Spitz showed that early nurturing and stimulation are essential to child development, and he was not alone in this view. At the time, the field of psychology was dominated by the theory of "behaviorism," which proposed that all our actions, from the simplest smile to the most sophisticated chess move, are learned through reward and punishment, trial-and-error interactions with other people and objects in the world. Babies, according to this view. are born as "blank slates," without predispositions, and infinitely malleable through parental feedback and tutoring. John Watson, the founder of modem behaviorism, even went so far as to claim:

Give me a dozen healthy infants, well-formed, and my own specified world to bring them up in and I'll guarantee to take any one at random and train him to become any kind of specialist I might select—doctor, lawyer, artist, merchant-chief, and yes even beggar-man and thief. regardless of his talents, penchants, abilities, vocations, and race of his ancestors.

No doubt Watson overstated his case, but such emphasis on early environment eventually led to the establishment of important social programs like the welfare safety net and Head Start. If children are so greatly malleable. then the best way to ensure a great society is by improving the environment of its youngest members.

Another experiment, however, proves that babies really do imprint on auditory experiences while still in the womb. In this case, mothers were asked to read a particular story aloud, twice a day, during the last six weeks of pregnancy. The story was by Dr. Seuss again, this time The Cat in the Hat, and it was estimated that the babies spent a total of about five hours listening to it in the womb. Shortly after birth, they were tested to see whether they preferred listening to their mothers read this story or another one. The King, the Mice, and the Cheese. These newborns sucked more to hear The Cat in the Hat, showing that they both remembered and preferred a story they'd heard only in the womb. Still another study showed that newborns prefer the sound of their mother's voice as it is actually heard in the womb—in the same muffled, deeper tones produced by the sound traversing her body (which researchers were able to mimic by filtering out frequencies above 500 Hz)—to the way it sounds outside the womb.

So fetuses not only begin hearing well before birth, what they hear in the womb has a surprisingly large impact on them. In addition to their mother's before birth. One of their favorites is her heartbeat, a steady, comforting companion that they hear from the onset of hearing to the moment of birth. Newborns are known to be calmed by the sound of a maternal heartbeat, and one study even found that repeatedly playing the recording of a mother's heartbeat into the incubators of preterm babies improved their mental development as measured at two years of age.

If babies can remember The Cat in the Hat after several repetitions in utero, it is easy to imagine other familiar sounds they develop a liking for— a lullaby sung every night to an older child, a currently popular song on the radio, or (as in the case of my second baby) the fan inside a desktop computer. One British researcher found that newborns whose mothers watched a particular soap opera during pregnancy stopped crying when they heard the show's theme song, whereas babies whose mothers hadn't watched the program showed no reaction to the song. Newborns apparently have a keen memory for their prenatal auditory experiences, and hearing those familiar sounds is yet another way of smoothing their transition to postnatal life.

Unfortunately, one stimulus that doesn't appear to register prenatally is Daddy's voice. When tested, newborns have not been found to be capable of recognizing their own father's voice better than that of a strange male. This result is surprising, because as we have already seen, lower tones penetrate the womb better than high ones. It may be that fetuses can't learn their father's voice because it is masked by all of the other loud, low-frequency tones coming from the mother's own body—her heartbeat, blood flow, and stomach rumblings. This masking, together with newborns' strong familiarity with their mother's voice, means that they generally prefer female over male voices, even a strange woman's voice over that of the father.

But fathers need not despair. Within a few weeks of birth, the baby will know and prefer his voice to that of other men. It is also quite possible that if a father made a special effort to speak loudly to his wife's stomach every day over the last month or two of pregnancy, his child would know his voice from the moment of birth. The experiment simply hasn't been done yet.

The similarity between different vertebrate embryos is indeed remarkable. Since the early 1800s, embryologists have been struck by the parallel between early development in various animal species and their evolutionary relationship, a resemblance conveniently abbreviated by the saying "ontogeny recapitulates phylogeny." Of course, each of us does not really pass through a "lizard" stage on our way to a fully developed human form. But it is true that animals who are more closely related in terms of evolution will resemble each other for a longer period of embryonic development. At four weeks, a human embryo is barely distinguishable from any other vertebrate embryo—bird, reptile, or mammal—but by six weeks it resembles only other mammalian embryos, and by seven weeks, only certain primate embryos. such as monkeys.

The similarity between ontogeny and phylogeny shows that the strategy of early development has been highly conserved in evolution. This makes sense. if you think about the precise timing and series of events necessary to turn a single fertilized egg into many different complex organ systems; it's simply much easier to add changes at the end of a common developmental sequence than to alter things from the outset. A slight change early in neurulation, for example, could invalidate all kinds of later, subtly timed cues, throwing off the whole process of brain formation. (Just such a problem occurs in spina bifida, a relatively frequent condition in which part of the spinal cord is not fully enclosed because of a defect in the early neural tube.) It has been much easier for evolution to take an existing structure, like a forelimb, and turn it into a wing, or a primate cerebral cortex, and enlarge it into the human cortex, than to start with a whole new game plan for each species. Evolution proceeds through the selection of random mutations, and the later in development such a change occurs, the likelier it is to produce a viable offspring than a horrible mistake. Indeed, this is why miscarriages are more common in early pregnancy.

It is from a public health standpoint, rather than from knowledge of indipercent of pregnant wompr. i^ percent of pregnant women in the United States reported drinking alcohol in the month preceding the survey, and 3 percent admitted to at least one binge. (Alcohol consumption is notoriously underreported in this kind of survey.) Prenatal alcohol use is thought to be responsible for at least 4,000 cases of mental retardation in the United States each year and perhaps ten times that number of children with mild learning or behavioral problems. (Because the milder effects of prenatal alcohol generally do not show up until several years after birth, one wonders how often these e effects are attributed to other factors.) Society pays a heavy price for alcohol consumption during pregnancy, which is why the U.S. Congress began requiring warnings about FAS on alcohol bottles in 1989. Although alcohol consumption during pregnancy had declined substantially in the 1980s, it actually increased between 1991 and 1995.

Most women also realize that smoking is bad during pregnancy. Unfortunately, it can be a very hard habit to give up, even for the best-intentioned parent. Smoking is not as detrimental to fetal brain development as heavy alcohol drinking, but it acts on many other organ systems. like the heart and lungs, that compromise the baby's health in a lasting way. Babies born to heavy smokers are substantially smaller than babies born to nonsmokers, averaging about half a pound lighter. In fact, smoking is one of the leading preventable causes of low birth weight, since 25 percent of women are estimated to smoke during their pregnancy. (These babies are born smaller, regardless of the amount of weight the mother gained during pregnancy, so their low birth weight is not due simply to the possibility that their mothers ate less than nonsmokers.) Smoking also increases the risk of miscarriage and premature birth caused by problems with the placenta. Both prematurity and low birth weight increase a child's chances of mental or neurological impairment.

When a pregnant mother smokes a cigarette, nicotine rushes into the fetal circulation and dramatically alters the baby's breathing movements. with periods of apnea (cessation of breathing) alternating with periods of extra-rapid breathing. Babies whose mothers smoked during pregnancy have a greater risk of sudden infant death syndrome (SIDS), which may be related to this altered breathing pattern in utero.

Several long-term studies of children bom to mothers who smoked during pregnancy have suggested that their brain development and function are compromised. In various reports, prenatal smoking has been linked to deficiencies in newborn sucking ability, in language and motor skills in one- and two-year-olds, in hyperactivity and auditory attention in four-to-seven-year-olds, and in learning ability in seven-to-eleven-year-olds. Two other studies also link it to attention-deficit hyperactivity in six-to-seventeen-year-old boys and to mental retardation of otherwise unknown cause. Some of these results are controversial and may be complicated by the fact that, on average. smokers tend to consume more alcohol and to be of lower socioeconomic status than nonsmokers. Overall, however, the evidence to date does suggest that heavy smoking during pregnancy has long-term effects on cognitive abilities that are probably due to compromised brain development in the womb.

Caffeine crosses the placenta and may even concentrate in the fetal circulation. Concern about its effect on fetal development stems from animal studies, where it has been found to be teratogenic when fed to pregnant rats in high doses; a dose equivalent to 150 cups of strong coffee per day causes malformations in rodents such as missing limbs and digits. However, caffeine does not appear to be a teratogen in h humans. The average pregnant woman is estimated to consume 144 milligrams per day of caffeine, which presents no danger to a developing fetus. Even women who consume fairly large amounts of caffeine (more than 400 milligrams per day) during pregnancy do not increase their risk of having a baby with a congenital malformation. And despite old wives' warnings, prenatal exposure to caffeine does not stunt fetal growth; nor, according to one study of seven-year-olds whose mothers drank caffeinated beverages during pregnancy, does it have any effect on a child's later IQ.

As we learn more about maternal hormones and their influence on the developing brain, scientists are beginning to propose actual biological mechanisms for the kind of folk prophecies that have been around for ages. One recent study, for instance, suggests that a child's shyness is determined, in part, by maternal hormone fluctuations during gestation. Researchers who interviewed several thousand preschoolers in both the United States and New Zealand noted a significant relationship between the incidence of extreme shyness or inhibition (children who seem particularly fearful, anxious, or withdrawn in the presence of a stranger) and the amount of daylight their mothers were exposed to at midpregnancy. Thus, in the United States, only 12 percent of children born in October-November-December were rated as highly inhibited, compared to nearly i8 percent of those born in April-May-June. In New Zealand, where daylight hours are reversed, children showed the opposite pattern, with more shy children born in October-November-December than in April-May-June. Because the production of certain hormones, like melatonin, is known to fluctuate with the amount of daylight in each season, the researchers propose that such substances may subtly alter brain development during a critical period at midgestation, when massive numbers of neurons are migrating to form the basic architecture of the cerebral cortex. (It is also possible that other seasonal differences, like changes in women's diets, physical activity, or exposure to colds and flu. mediate this relationship.)

Of its several advantages, "birtn stress" has been found to be especially beneficial for a newborn's breathing. Compared with babies born by C'Section, vaginally delivered babies are quicker to take their first breaths; their blood oxygen levels rise more rapidly after birth; and they are less likely to suffer any of a number of respiratory problems in the first few hours of life. Even among babies delivered by C-section, those who undergo several hours of labor before delivery do much better than those delivered prior to the onset of labor, although not as well as vaginally delivered babies. Higher catecholamine levels explain much of the respiratory advantage of "stressed" babies, because these hormones are known to help absorb some of the excess liquid in the lungs at birth and to promote the release of lung surfactant detergent-like molecules that are necessary for gas exchange through the lung's tiny grapelike air cells, or alveoli. Other stress hormones, such as cortisol, probably also contribute to this last-minute lung maturation. Finally, vaginal delivery further aids the onset of breathing in a purely mechanical way: by helping squeeze some of this extra liquid out of the lungs as the baby's chest is compressed during passage through the birth canal.

Higher catecholamines also benefit vaginally delivered babies in other ways. Because catecholamines speed up metabolic rate, vaginally delivered babies are better able to maintain their body temperature, and they have larger reserves of glucose and other energy sources than C-section babies. They are also better adapted neurologically to life outside the womb, judging by their higher scores on tests of reflexes, muscle tone, and sensory responses during the first two days of life. Considering all these benefits of labor stress for the baby, some obstetricians now recommend that women planning to deliver by C-section first undergo at least the early stages of labor before surgery.

Of all the advantages of labor, some of the most intriguing are those that affect the baby's nervous system. There is some evidence that contractions even of the prelabor Braxton-Hicks type, promote brain development in sheep. Perhaps the additional touch and movement stimulation provided by contractions helps refine synaptic connections or promotes myelination during late gestation. Then, once true labor and delivery ensue, a baby's high catecholamine levels potently stimulate the nervous system. In adults, a large surge of adrenaline is highly arousing and can lead to a feeling of well-being. Catecholamines appear to have the same effect on newborns, who are more alert during the first two hours of life than for many days thereafter.

Touch plays a very special role in the life of young babies. Because it is so well developed at birth, it provides these brand-new arrivals more detailed access to their fascinating new world than any other sense. Touch is obviously essential to babies' sensory-motor development, but it also has a surprisingly potent influence over their physical growth, emotional well-being, cognitive potential, and even their overall health, because of some fascinating effects on their immune function.

[...]

Newborn rats that are handled for just a brief period each day by human experimenters show all kinds of hormonal and behavioral advantages that stay with them throughout life. Handled rats are less fearful, have more brain receptors for benzodiazepines (anxiety-reducing tranquilizers that mimic the action of a natural inhibitory neurotransmitter, GABA), less degeneration in old age of the hippocampus (a critical memory-storing area of the brain), and correspondingly better cognitive performance as they age. All of these improvements can be traced to the fact that neonatal handling permanently reduces the reactivity of rats' stress response systems. Handled animals show the normal hormonal responses to stress (see Chapter 3), but their corticosteroid levels do not rise as high as those of nonhandled animals, and they recover more quickly. Since prolonged elevation of stress hormones can be quite damaging to many organs of the body, including the brain, a better-modulated stress-response system is advantageous to both an animal's health and its mental faculties.

Perhaps the most interesting feature of this handling effect is that it works during only the first ten days or so of a rat pup's life. Pups who are handled only after this critical period do not show the same permanent advantages. Of course, human handling is not a natural stimulus for a rat, but recent research has found many of the same benefits for those rat pups who receive greater tactile stimulation from their mothers. Rat dams, like human mothers, vary in their styles of nursing and contact, and it turns out that those that lick and groom their pups more during nursing induce the same lasting benefits in their offspring. They have a better-modulated stressresponse system, including changes in brain neurochemistry that make them less fearful in novel situations.

Other animal studies have focused on the effects of maternal separation on infants' growth and immune function. Infant monkeys become very distressed when their mothers are removed; their stress hormones rise during even brief separations, while longer separations are known to suppress their immune system. This suppression reverses if mother and infant are reunited within ten days, but if they remain separated for longer than that, the effect appears to be permanent; the offspring continue to show reduced immune function as late as six years of age. Rat pups, too, derive an immune benefit from early touch, since handled animals produce higher levels of antibodies in response to an immune challenge than nonhandled rats. As in monkeys. short-term maternal separation raises rat pups' stress hormone levels and is also known to inhibit the release of growth hormone and to suppress cellular growth and differentiation. These effects, which are limited to the first three weeks of a pup's life, can be prevented by firm human stroking, suggesting that it is the mother's actual touch and contact—as opposed to, say, her warmth or nursing—that especially promote infant growth.

One study offers particularly provocative evidence of the benefits of vestibular stimulation. These researchers exposed babies, who ranged in age from three to thirteen months, to sixteen sessions of chair spinning: Four times a week for four weeks, the infants were seated on a researcher's lap and spun around ten times in a swivel chair, each spin followed by an abrupt stop. To maximize stimulation of each of the three semicircular canals, the spinning included one or two rotations in each direction with the babies held in each of three positions: sitting, with the head tilted forward about 30 degrees. and side-lying on both left and right sides. Not surprisingly, the babies loved this treatment. They usually babbled or laughed during the rotation and became fussy during the thirty-second rest period between spins. In addition to this "trained" group, there were two groups of control infants, one that received no treatment, and one that came in for the same sixteen sessions but only sat on the researcher's lap in the swivel chair; they did not get to spin.

The results were striking. Compared with both control groups, the babies who were spun showed more advanced development of both their reflexes and their motor skills. The difference was particularly marked for motor skills like sitting, crawling, standing, and walking. In fact, the study included a set of three-month-old fraternal twins, of whom one received the training and the other did not. By the end of the study, when they were four months old. the twin who had experienced the vestibular stimulation had mastered head control and could even sit independently, while the unstimulated twin had only just begun to hold his head up.

[...]

Vestibular stimulation can have a profound impact on a baby's overall behavioral state. Young babies tend to go through periods when their behavior is best described as "disorganized"—they flail their limbs, tense up their hands and face, and cry in an insistent, high-pitched way. (Toddlers and preschoolers have similar periods of disorganization, otherwise known as tantrums, but they are fortunately far less frequent.) Parents will do just holds him over her shoulder, and gently jiggles him, he soon becomes "organized" again; his crying stops, his body relaxes, and for a brief time, he is highly alert—looking intently at the lamp behind Anna's back, then at the bright picture on the wall, and finally, when she moves him to a cradling through vestibular stimulation show greater visual alertness than babies comforted through vestibular stimulation show greater visual alertness than babies comforted in other ways. It's during these periods of quiet alertness that babies do their best learning, when they can most effectively absorb information about the world around them.

Continued vestibular stimulation has a very different effect: It decreases a baby's level of arousal. After Anna carries him around for a while, Timothy gets sleepy again and eventually dozes off. This, too, is beneficial to his maturing brain, which does a lot of important growing during the sleeping hours.

In the old days, extensive parental contact was discouraged because of fears about injury or infection. Now, however, some hospitals are encouraging parents to spend up to several hours a day holding their preterm infants, preferably upright and skin-to-skin against their bare chest. This approach has been dubbed "kangaroo care" because of its resemblance to early-life marsupials, which are born prematurely but kept warm and nourished in the maternal pouch.

Studies have shown several advantages of kangaroo care. Babies are better able to maintain their body temperature, so they do not use any excess energy during kangaroo care with either the mother or father. They sleep better, cry less, breathe more regularly, breast-feed longer, gain weight faster, and are discharged from the hospital earlier than comparable preterms who do not have skin-to-skin contact with their parents. Equally important are the benefits to parents, who bond sooner and express greater confidence about parenting when they "kangaroo" their preterm baby. And the greatest advantage of kangaroo care is in promoting breast-feeding, which can be very difficult to establish when babies are born prematurely. Warm, safe, and comforted by the familiar maternal heartbeat, preterm babies nurse more and earlier when held for long stretches close to their mother's bare breasts.

Yet another approach for increasing preterm babies' touch experience is to add a massage to their daily routine. Infant massage has a long tradition in southern Asia, where gentle, systematic stroking and rubbing of the baby's entire body is considered an important part of daily infant care. Even in orphanages, Indian babies are treated to regular massages, and these children grow and develop remarkably well, especially considering their many other disadvantages. In the United States, several controlled studies have now shown that massage improves the health and development of babies compromised by various medical problems, including prematurity, prenatal cocaine exposure, and HIV infection.

For about an hour each day, nurses gently mb or stroke a preterm baby's entire body—face, shoulders, back, chest, arms, and legs—pausing between each region so the baby does not become overstimulated. (If the touch is too light, babies react aversively, as if they're being tickled, and do not experience the same health benefits.) This is often followed with gentle flexion and extension of all four limbs, providing proprioceptive stimulation. Preterm infants who receive these daily massages gain weight faster, perform better on neonatal behavioral tests, and, because of their more rapid progress, are able to leave the hospital earlier than comparable preterms who do not receive this stimulation. Such massaging also improves the development of touch itself; by the time they reached full-term age, preterms given the massages proved to be more responsive to touch than preterms who had not. (But both groups were less sensitive than normal full-term babies.) Most encouraging are later cognitive effects of this early massage therapy; in one study, preterms who had been given the massages performed better on tests of visual recognition at six months of age than comparable control preterms.

Preterm babies are not the only ones who can benefit from daily massage. In one recent study, full-term four-month-olds were given an eight-minute massage shortly before being assessed for "novelty preference," a procedure that tests early memory and sensory discrimination skills. Compared with control babies, who were simply entertained by the experimenters with a red toy for the eight minutes preceding the test, massaged babies were significantly better at detecting when one auditory-visual stimulus changed and a new one appeared. As we will see in Chapter 13, novelty preference actually predicts later IQ better than any other infant skill, suggesting that regular. early massage may have important cognitive benefits for babies of all gestational ages.

Olfactory recognition may also be the first step toward human bonding and attachment. As we've seen, newborns quickly learn and prefer the scent of their own mother or other caretaker. Nursing babies clearly have the richest olfactory experience, being bathed several times a day in the odors of their mother's milk and areolar secretions. Nonetheless, bottle-fed babies also can learn their parents' scents rather rapidly, depending on the amount and closeness of their contact. After the breast, a caregiver's neck is probably the most potent source of olfactory input, since it is often uncovered and close to a baby's nose when he is being held upright.

Young children are also known to recognize and prefer the odors of their own siblings, based on some more T-shirt sniffing experiments. Children as young as three years of age can correctly identify the odor of their own sibling, which undoubtedly contributes to the development of this special bond.

[...]

Like prenatal smell, a fetus's taste experience in the womb may bias his or her later behavior—in this case, influencing food preferences. This has been clearly demonstrated in animal studies. For instance, baby rabbits whose mothers were fed juniper berries during pregnancy clearly preferred foods containing the aromatic juniper flavor when tested at the age of weaning. In another report, adult rats that had been exposed to apple juice before birth by amniotic injection showed a greater preference for apple juice than rats exposed to control or saline injections. And more worrisomely, adult rats that were exposed to alcohol in utero showed considerably more preference for it than rats that had not been exposed to alcohol before birth. If the same kind of "memory" occurs among the children of alcoholic mothers, it may in part explain why alcoholism tends to mn in families.

The fact that babies begin to taste before birth thus has important developmental consequences. Not only does it affect the formation of taste pathways and preferences, it probably also acts, like smell, to help babies recognize and find comfort with their mothers after birth, since many of the same dietary flavors that make their way into a woman's amniotic fluid will also be present in her breast milk.

One reason why researchers have had such a hard time replicating the composition of breast milk is that it isn't a fixed commodity. No two women's milk is identical, nor is the composition of any one mother's milk constant at all times; it varies with the amount of time that has elapsed postpartum, gradually changing in composition to match the baby's changing nutritional needs. It also varies with time of day, with the thinnest milk (the lowest tat content) being produced early in the day and the richest produced in the evening. Finally, it varies with the mother's diet, providing breast-fed infants a rich medium for experiencing many different flavors in early life.

Breast-fed babies clearly like this flavor variation. In one experiment. three-to-four-month-old babies were found to suck longer and consume more milk after their mothers had ingested a garlic pill than when they ingested a tasteless placebo tablet. The difference was most pronounced two to three hours after mothers ate the pill, the same period when the garlic odor of their milk was the strongest. Although garlic-flavored milk may not sound very appealing, young babies apparently love it. If the mothers repeatedly consume the garlic tablets, however, their infants no longer suck for extra periods of time. Like adults, they seem to get bored with tasting the same thing over and over and prefer some variety. Fortunately, there are plenty of possibilities. Women's milk is also known to be altered by ingesting vanilla, mint. and cheese, and probably most other distinctive flavorings, herbs, and spices.

Variation in breast milk flavors may play an important role in taste development itself. Even before a baby is exposed to any solid foods at all, he experiences all kinds of new flavors through his mother's breast milk, perhaps biasing his later taste preferences. Indeed, studies of rats, cows, and lambs have shown that when nurslings are exposed to distinctive flavors through their mothers' milk, they tend to prefer that flavor well after weaning.

Beginning about four months of age, the perception of detail takes another leap forward with the emergence of hyperacuity: the ability to discriminate features that are up to ten times finer than the size of the photoreceptors should theoretically permit. It is this ultrafine discrimination that allows us, for instance, to see a very slight glitch in an otherwise straight line. even though the size of the glitch is below our eyes' limit of resolution. It is not yet known how our brains perform this remarkable feat, but it is generally agreed that the necessary processing takes place in the cerebral cortex. Babies show rapid improvement on hyperacuity tests between ten and eighteen weeks of age, in accordance with the massive maturation of the visual cortex over the same period.

[...]

In the second month of life, babies exhibit another striking visual behavior that is related to the externality effect. It is called obligatory looking, and as the name implies, babies this age may fixate on a single object, sometimes for thirty minutes or more. The reason their gaze gets stuck is because this is when the visual cortex first begins exerting control over brain-stem visual centers, the net effect of which is to inhibit babies' habitual eye movements toward their peripheral visual field. So even though Ginna, now six weeks old, wants to look away from that bright lamp in front of her, an awkward struggle between her cortical and subcortical visual centers prevents her from doing so. Poor thing!

One trick our brains use to figure out the location of a sound is to compare the time it takes to reach each ear. For example, sound waves emanating from a wind chime located to your right will reach your right ear a few milliseconds earlier than they reach your left ear, and the brain uses this small timing difference to compute exactly how far to your right the chime is located. Researchers have capitalized on this timing difference to test sound localization, using a special experimental trick known as the precedence effect: by playing the same sound, slightly separated in time, out of two loudspeakers located on either side of a subject, they produce the illusion that it is located to the left or the right. When older children or adults hear such a sound sequence—with, say, the right speaker preceding the left by several milliseconds—they perceive the sound as coming from the right. Newborns, however, fail miserably at the precedence effect. They are unable to use timing differences to calculate the location of a sound until about three or four months of age, when the cerebral cortex becomes fully engaged in the process. (But newborns can detect differences in loudness between the two ears, which is the cue they use to localize sounds in the horizontal plane.)

Believe it or not, it is only relatively recently that scientists have begun to appreciate the importance of babies' earliest motor activity. In the first part of this century, most researchers championed the view that motor development is largely innate, or "hard-wired." Struck by the remarkable consistency of skill acquisition, they argued that motor development depends solely on a fixed process o{ neuromuscular maturation (as their theory came to be called). with little role for practice or experience.

One 1940 study was often cited as supporting the neuromuscular maturation theory: an analysis of walking in Hopi Indian babies. Traditionally, a Hopi baby spends much of her first year as a papoose—strapped to the mother's back in a cradleboard in which she can barely move. Despite this confinement, researchers found that Hopi babies reared in the traditional manner developed no differently from those reared in Western fashion, without the cradleboard: both groups of babies began walking at fifteen months, which is a little late but still within the normal range. Of course, the traditionally reared babies were not swaddled during all hours of the day; in the early months, they were removed for bathing and changing, while in later months, they spent several hours each day outside the cradleboard, and few were cradled at all beyond nine months of age. Nonetheless, this study convinced many early researchers that practice and muscular exercise are relatively unimportant in determining when motor skills emerge.

If motor restrictions have little effect, what about the opposite—intensive exercise in early life? As another way of testing their theory, researchers in the 1930s used identical twins for some remarkable training experiments. In each study, one twin was extensively helped and encouraged to practice at a particular skill—like rolling over, sitting, standing, stair-climbing, blockbuilding, tricycle-riding, or potty use—while the other twin received no special training. Despite their lengthy workouts, the trained twins were no more advanced in their motor skills than their siblings who spent their infancy in comparative leisure. This was particularly true for more basic abilities, such as walking and standing. So again researchers concluded that it is the fixed pace of neuromuscular maturation that determines a baby's motor progress; all the practice in the world isn't going to accelerate a particular motor skill if the baby's brain and muscles are not developed enough.

In fact, babies do improve their motor skills much as adults do—as a result of diligent practice. New skills, such as walking independently, don't suddenly emerge out of nowhere but gradually build out of prior, simpler abilities—kicking, standing, and walking with support—after weeks or months of trying. The only difference between infant and adult motor learning (aside from the fact that infants seem to crave the exercise more than most of us) is that babies can train themselves in a particular skill only when their brains are maturationally ready. In other words, practice is essential, provided it's done at the right time. Done too early, the necessary circuits simply aren't there to benefit from it. (Indeed, some researchers believe that premature practice can actually interfere with the acquisition of certain skills, either because it ends up training the wrong neural pathways or because the baby grows frustrated with trying to do something he has no hope of mastering at the time.)

Once again, it all comes down to understanding how the brain becomes wired up to perform certain tasks. Like each of the sensory systems, motor pathways are initially specified by genes—by innate signals that direct axons from the motor cortex, for instance, to grow down into the spinal cord, then stop at the appropriate level to innervate, say, the cervical motor neurons that control the hand. Though fairly specific, these initial connections are not precise enough to control all the skilled and elaborate movements of which Ethan will eventually be capable. Rather, motor pathways go through another stage of development in which they are refined through use; the more a particular pathway is activated during consistent, purposeful action, the likelier it is to be stabilized.

Even in adults, motor pathways can be modified with training. Recent brain-imaging studies have shown that when a person becomes skilled at a certain motor task, like a set sequence of finger movements, a larger area of the motor cortex is activated during the sequence than before it was well practiced. At the same time, repeated practice is also known to decrease the degree to which the cerebellum is activated during a task; although the cerebellum is critical during motor learning, it drops out once a skill is so well practiced as to require little concentration.

These kinds of imaging studies can't be performed in babies, but there is little doubt that the same sorts of changes take place in Ethan's brain as he masters each new motor skill. Motor learning involves a process of neural selection. The same movement can be accomplished by many different pathways and patterns of activity, but only some of these will be the most efficient. What practice does is to find, by trial and error, the few most efficient patterns and to strengthen and stabilize them, so that each time Ethan attempts a particular movement, he is increasingly likely to use the fastest, smoothest route.

Because it develops so early, this brain asymmetry appears to be largely innate. It is possible, however, that environmental factors begin operating even before birth. One hypothesis is that the right hand becomes more skillfull because it has greater freedom to move in the womb. About three-quarters of all fetuses spend the last several weeks of gestation with their right arm facing out—toward the mother's abdominal wall. This arm has more space in which to move than does the left arm, which keeps running into the mother's spine. This may lead to differential growth and wiring of the hand areas on each side of the motor cortex. However, studies to test this prenatal position hypothesis have thus far proven inconclusive.

Another possibility is that the right hand tends to become more skillful because of an innate preference in head orientation. It turns out that most newborns favor turning their head to the right. Because head-turning triggers the asymmetric neck reflex in newborns—a brain-stem response in which both the arm and leg extend on the same side that the head is facing, while the opposite limbs flex (see Figure 6.3)—babies spend a lot more time looking at their right arms than their left, a posture that will preferentially promote hand-eye coordination on the right side. In fact, studies have shown that babies who have just begun reaching tend to favor the same hand as their head orientation, although they don't necessarily preserve this hand preference later. It's not known why babies favor a right-head orientation, but it may have to do with the fact that most parents hold their babies in their left arm, regardless of their own handedness.

Finally, there is one school of thought (though not a very popular one among left-handers) that states that right-handedness is the "normal" or default developmental pathway and that all left-handedness is the result of some kind of pathology. The logic here is that the brain is genetically destined to be right-handed, but that any kind of prenatal or birth-related damage to the left hemisphere is going to switch hand control to the right hemisphere—the left hand. Left-handedness is indeed much more common among people with disorders related to brain damage, such as epilepsy or mental retardation. Some studies have also found that babies experiencing more difficult deliveries are likelier to have altered patterns of hand use. But it seems very unlikely that all left-handers could have experienced some kind of covert left-hemisphere damage that left them cognitively unaffected in other respects. Rather, most researchers believe that there are two categories of left-handed individuals: the majority, whose handedness is largely inherited, and the minority, whose handedness is due to some kind ot pathological event before or during birth.

In fact, contrary to all of the early anecdotes claiming that practice has no effect on the onset of walking, one carefully controlled study has shown that special exercise can indeed accelerate it. In this study, a group of newborns were given just ten minutes per day of "practice walking." Every day between one and nine weeks of age, the baby would be held upright by a parent, with his feet on a table, and allowed to exercise his stepping reflex. Two additional groups of babies received, respectively, either no exercise but weekly testing of their walking reflex, or passive exercise, in which a parent would altenately pump the baby's legs and arms while he was lying down. Compared with these two control groups, whose walking reflex declined during the eight weeks, the babies who were actively exercised maintained their walking reflex and even took more steps with each passing week. Moreover, when it came time to walk independently, the actively exercised babies achieved this milestone a full month earlier than the other two groups of babies, and two months earlier than an additional group of babies whose walking reflexes were not even tested during those early weeks.

How does early practice accelerate later walking? Probably not by affecting the corticospinal tracts, which mature too late to benefit from exercise during the newborn period. Rather, it is likely that it strengthens babies' muscles and tunes up their more precocious neural pathways, such as the circuits involved in balancing upright. The fact that the amount of acceleration is modest—babies in this study walked at around ten months of age (still within the normal range) and not at two, or five, or even eight months— shows that motor development is not massively plastic; basic neural and bodily development does set a lower limit on when a child can begin to walk. Nonetheless, learning to walk, like all other motor skills, takes practice, and even in the earliest weeks of life, motor activity influences in a lasting way how a baby's brain and muscles develop.

While practice is important in learning to walk, one type of exercise does not help and even poses a significant danger for babies: the use of infant walkers. One researcher found that babies who spent about an hour per day in their walkers, beginning around four months of age, did not walk any earlier than babies who had never used walkers, while others found that babies who used walkers for about two and a half hours per day were actually delayed in walking and other gross milestones. The problem with walkers may be that they make it too easy for babies to move around. They can explore and satisfy their curiosity without developing their balance or locomotor skills, so these abilities come more slowly. Another problem is that walkers block babies' view of their feet, and this visual feedback is important when babies take their first independent steps.

Memory is not a single entity but a patchwork of several different forms of information storage that emerge progressively with the maturation of different brain circuits. Babies begin life with a primitive yet very useful set of memory skills; lower parts of the brain can store information, but it is at an automatic level, beneath consciousness, and lasts for relatively short periods of time. Then, starting at eight or nine months of age, they show signs of a more flexible, deliberate type of information storage, the first inklings of memory as we more commonly think of it. Memories then grow longer and increasingly conscious throughout the preschool years until finally, during early elementary school years, children become aware of their own memory skills and begin to use them in a truly mature way—to intentionally study and acquire new information.

The development of memory is both fascinating and fundamental to every other aspect of cognitive growth. It's fascinating because in a very real sense, we are the sum total of what we can remember; memories create the mental continuity that gives each of us a coherent sense of who we are and what we have uniquely experienced. Watching memory dawn in a young child is almost like seeing consciousness gradually emerge out of the fog bank of early experience. Memory development is also critically important, because the brain's enormous capacity to store information is what makes every kind of learning possible. Whether it's bonding with Mother, recognizing Aunt Betsy, mastering crawling, associating words with objects, or figuring out that water is wet, every mental advance depends on the brain's ability to file away experience and then use this stored information to act with greater wisdom and efficiency. Memory is truly the cornerstone of intellectual growth, the brain's sole means of acquiring knowledge, so it is not surprising that even in infancy, it serves as a marker for later intelligence. At the same time, memory flexible skill that can improve with practice. By understanding how the brain's various storage systems develop, we may be able to optimize the several mnemonic skills that are essential to intellectual growth.

[...]

The first type of learning to emerge is habituation, the progressive decline in responding that psychologists find so useful for probing babies' minds. Almost as soon as they begin to hear, fetuses habituate to a repeated acoustic stimulus, a loud sound or vibration applied close to the mother's abdomen. Initially, the sound triggers a dramatic startle response—large movements of the baby's limbs or torso that can be observed under ultrasound—but if the stimulus is repeated every twenty seconds, a fetus will respond less and less vigorously until finally he ceases altogether. A few babies show habituation as early as twenty-three weeks of gestation, and by twenty-nine weeks, all healthy fetuses can do it.

[...]

Habituation is not the only form of learning that babies exhibit before birth. The same fetal reflexes have also been shown to adapt by a process known as classical conditioning, a learned association between stimuli. Most of us learned in school about Pavlov's famous discovery—that dogs will begin to salivate to the sound of a bell if they have repeatedly heard it rung with the delivery of food. A human fetus can similarly learn after repeated pairings that a sound will signal a vibratory stimulus or, perhaps more meaningfully, that a brief clip of music will relax its mother. In the latter case. pregnant women were asked to consciously relax whenever they heard a particular piece of music, say, Beethoven's Moonlight Sonata. As any pregnant woman knows, fetuses are most active when their mothers are the most relaxed, but the mothers in this experiment soon noticed their babies beginning to move to the music alone, even before they had a chance to relax themselves. Then, after birth, the same music was found to have an especially calming effect on the babies. Classical conditioning has been reported in fetuses as early as five and a half months of gestation and remains an important form of learning throughout life.

[...]

The best-studied operant procedure is so simple, and babies enjoy it so much, that you might want to try it with your own child. It is called mobile conditioning. A young baby, like bright-eyed three-month-old Robert, is laid in a crib, facing up at an attractive mobile of five dangling, brightly colored blocks. The mobile itself is pretty exciting, but it gets even better after the experimenter ties a ribbon around one of Robert's ankles and attaches the other end to the mobile's crossbar. Suddenly, Robert is kicking two or three times more frequently than before the ribbon was attached (established as the baseline period). He has discovered that he can actually move the mobile by moving his leg and is simply delighted at this newfound power.

This is the learning phase of the experiment. Memory-testing comes later, when the mobile is reinstalled in his crib but not connected to his foot. Though he hasn't seen it for three days, Robert starts kicking as soon as he sees the mobile, showing that he remembers, at some level in his nervous system, that this behavior will prove rewarding. (It won't, of course, since there is no ribbon this time, so this phase of the experiment can extinguish his increased kicking if allowed to continue more than a couple of minutes.)

Babies as young as two months of age can learn to move the mobile and remember how to do it for a day or two. Between two and six months, they master the task more and more quickly and also remember it for longer and longer periods; three-month-olds remember up to a week and six-month-olds remember up to two weeks after a single training session. They also steadily improve in the amount of detail they can remember. For instance, if the mobile is changed in some way—say, by replacing the blocks with five balls—^Robert will still kick to get it moving. By six months of age, though, he is much more particular and won't kick at the accelerated rate to anything but the identical mobile he trained on. He won't even kick if his crib is moved to a different room or if its distinctive bumper is replaced by another pattern. The fact that young babies are sensitive to the exact environment or context in which they learn something is quite remarkable and shows that they learn not only how to obtain a reward but where certain actions will be reinforced, like rooting for milk only in their mothers' arms, or smiling only when making eye contact.

Like many other areas of development, memory generally matures more rapidly in girls than boys. Beginning in the womb, female fetuses are known to habituate to auditory stimuli about two weeks earlier than males. After birth, they are more advanced at visual habituation. Toward the end of the first year, girls are about a month ahead in tests of short-term, explicit memory, like remembering, after a few seconds' distraction, where they just saw a toy being hidden. Girls also outperform boys on a test of long-term implicit memory, the "concurrent discrimination" task... in which they must learn, over many repetitions and for several pairs of objects, which of the pair conceals a tasty Froot Loop. Between one and three years of age, girls make fewer errors on this task than boys; after that. both sexes perform comparably. Finally, females tend to perform better on tests of verbal recall, like remembering details about recent events, a difference that emerges in the fourth year and persists into adulthood.

Studies of infant monkeys have provided some clues to the neural basis for these gender differences. Once again, testosterone appears to be the culprit. Testosterone levels surge in male monkeys early in gestation and remain elevated until three or four months after birth. By six months of age, they are back down to around female levels. (They then surge again at puberty.) Males' performance on the concurrent discrimination task parallels these changes in testosterone, since it is notably poorer than females' at three months but equivalent by six months of age. Even among different three-month-old males, those monkeys with the highest testosterone levels show the poorest memory performance and vice versa. But the most convincing evidence that testosterone influences the development of memory-storage mechanisms in the brain comes from experiments in which hormone levels were manipulated, either by castrating young male monkeys or by injecting testosterone into young females whose ovaries were removed. As predicted, the castrated males remembered which objects were paired with the reward (in this case, a banana pellet) better than normal males and comparably with females of the same age, while the females injected with testosterone performed more poorly than control females.

The testosterone surge in humans lasts somewhat longer than in monkeys, not reaching its nadir until the end of the first year, which may explain why boys are generally slower learners than girls during infancy and early childhood. Testosterone appears to slow cellular development in certain cortical regions, including the inferior temporal cortex, an area of the visual system known to be involved in the concurrent discrimination task and which is both Structurally and functionally more mature in female infant monkeys than in males.

Psychologists have tested memory performance in people all over the world and found that those who have completed at least a few years of formal education score higher than those from the same culture and economic status who did not attend school; and the more years completed, the better the performance. Where formal schooling especially helps is in learning memory strategies, deliberate tricks like verbal rehearsal, information clustering, and note-taking that children use to make it through years of quizzes and final exams.

But what about the years before formal schooling begins, when the basic neural circuitry underlying conscious memory storage is still being laid down? Is there a critical period for memory development during these earlier years? We know that children begin using their memory in a deliberate fashion as early as three years of age. In one study, three-year-olds (but not two-year-olds) proved better at retrieving a hidden toy dog if the experimenter explicitly asked them, at the time of hiding, to "remember where the dog is" than i he simply instructed them to "wait here with the dog" while the experimenter briefly left the room. By three years, kids already figure out tricks like keeping their hand on the hiding place or forcing themselves to stare at it throughout the forty-second waiting period, to help prod their memories.

That children are aware of their memories at such an early age suggests that it may be possible to improve such strategies, if not memory itself, well before they enter elementary school. Indeed, there's good evidence from laboratory studies that children as young as four can learn strategies like sorting and naming that improve their ability to recall words or objects. Even more intriguing is the role parents can play. It is known, for instance, that three-year-olds whose mothers place greater demands on their memories—who more frequently question them about past events or probe their growing body of general knowledge—perform better on tests of recall than children whose mothers place fewer such demands on them. By focusing children on the important facts—the who, what, when, where, how, and why issues—parents can teach their children the requisite narrative skills—how to think about events in terms of time and causality—which is ultimately how we recall facts and events later on. Perhaps this is why young children love to be told stories; it's as if they instinctively crave examples by which to hone their own narrative skills.

Thus it does appear that memory development can be influenced by practice. The more a child is challenged to use her memory, even early on, the better it is likely to serve her later in life. The fact that memory skills can be molded by experience—even at an age when the basic neural pathways for information storage are still being laid down—suggests that the early years may indeed constitute a critical period for establishing a lifelong arsenal of memory skills.

Babies first bridge the gap between sounds and meaning as early as nine or ten months of age. They learn the names of family members and pets, the meaning of no! and perhaps a few general labels like shoe and cookie. By his first birthday, the average child understands around seventy words, mostly nouns like people's names and terms for objects, but also certain social expressions, like hi and bye-bye. Of course, he cannot say nearly that many. The median number of words spoken by a one-year-old is six, but many say none at all, and a few speak up to fifty. There's typically about a five-month lag between the time a toddler can understand a certain number of words and when he can actually speak that many.

New words accrue slowly between twelve and eighteen months. Nathan picks up a few nouns and expressions each month—spoon, blankie, nose, milk, up, allgone—trying each out for several days and often dropping them as he moves on to the next. But then, all of a sudden, his vocabulary hits critical mass: he starts saying new words every single day—car, cup, kitty, flower, plane, birdie, teeth, keys, hair, light, foot, let's go, ball, kiss, cracker, doggie, peekaboo, book, dance, water. Gramma, down, night-night, bath-time, eyes, ears. block, phone, bunny, hug, (com)puter, chair, tree, crib ... so many his mother can't keep up with the log she had begun keeping. Fifty is the magic number. Most toddlers' vocabulary explodes once they can say about four dozen words. Now they start adding one, two, or three new words to their speech every day, and their receptive vocabulary—the number of words a child understands—grows even more quickly. Between two and six, children are estimated to learn the meaning of a staggering eight words a day. That comes out to more than one new word every two hours they're awake, and they continue at this rate into the elementary school years. By the time a child is six, it's been estimated that he understands some 13,000 words, although he doesn't speak nearly that many.

[...]

There are really just two basic tricks of grammar used by all the languages of the world. You can create meaning either by adjusting the order of words or by changing the little pieces (known as inflections) that are tacked onto the ends of words (or beginnings, in some languages). For instance, the difference between "Big Bird is tickling Cookie Monster" and "Cookie Monster i tickling Big Bird" is conveyed both by the sequence of words—which proper noun is on which side of the verb—and by the form of the verb, since changing "tickling" to "tickled by" would exactly reverse the meaning of each sentence.

Toddlers begin to appreciate differences in word order before they begin combining words in their own speech. When sixteen-to-eighteen-month-olds were seated in front of a pair of television sets, each showing Sesame Street puppets acting out one of these two sentences, they looked more at the video corresponding to whichever sentence was playing on voice-over. Children thus appreciate the meaning embedded in word order at a very young age, an understanding that becomes quite useful when they begin speaking two-word phrases themselves, usually between eighteen and twenty-four months. Indeed, the vast majority of toddlers' first word pairs are in the proper order, minisentences such as: All dry. I shut. See baby. More cereal. Mail come. Our car.

There is no three-word stage in language development. Toddlers hang for several months in the two-word phase, still rapidly building their vocabularies. Then, beginning early in the third year, they swim into another linguistic vortex, this time the rapid accumulation of grammatical skills. It begins, of course, with the stringing together of more and more words, but the number can be three, four, or even more: J drive car-car. Plane go fast. That big dog^e nice. Now go outside. What the man doing on roof? Though correct in word order, these early sentences tend to lack most of the inflections and little function words—of, to, the, am, do, in, etc.—which is why they are called telegraphic, as if each word were at a premium. Before long, however, two-year-olds start adding little bits of grammar, and this too happens in a strikingly predictable way. English-learning children usually begin with the present participle {-mg) verb ending, as in, Where Mommy going! Then come prepositions such as in and on, followed by plural -s endings (cats), possessive -s endings (hers), articles (the, a), regular past tense endings {-ed), and third person present tense -s endings (walks), to mention just a few.

What's most fascinating about the way children learn grammar is that they are not simply doing it by trial and error; they are figuring out the actual rules for how different classes of words are combined. This means, first of all, that they intuitively grasp the distinctions between different parts of speech—nouns, verbs, adjectives, and so on. Before long, they figure out how to adjust and assemble these various parts to produce the precise meaning they intend. Say, for instance, that four-year-old Daniel is presented with a word he has never seen before: someone shows him a drawing of a birdlike animal and tells him that it is a "wug." If he is next shown a drawing of two such creatures and asked what they are called, Daniel will inevitably say "wugs." He already knows, without ever being taught, how to recognize a noun and make it plural. In fact, there are three different situations in which English grammar requires the use of -s endings, and children master all of them before the age of four, but in a distinct sequence: first, they figure out how to make plurals (dogs, cats, Elmo dolls); then, to use -s to indicate possession (dog's bone. Fluffy s yam, Elmo's doll); and last, to make present tense verbs to agree with a third person singular subject (The dog barks. Fluffy plays with yam. Elmo pees!). That children start adding these different -s's at different times proves that they can distinguish these separate parts of speech and the rules that apply to them; they are not simply imitating individual words or phrases from Mommy and Daddy.

Even more revealing are the mistakes young children make. Though parents may bristle at the sound of them, there's a good reason why older twos. threes, and fours come up with constructions such as: He gots a purple truck; She beed happy; Katie comed over; We swimmed at the pool. Each error is one of overgeneralization; the child takes an irregular verb—one of the roughly i8o in the English language whose past tense is not formed simply by adding -ed to the end—and tries to treat it like a regular verb. Children persist in these mistakes for several years, but the amazing thing is that they tend not to appear in the speech of very young children. In other words, young toddlers will often get a few of these irregular verbs right—like came, was, or has—before they figure out that a rule exists and begin substituting comed, beed, and gots. These errors continue, in spite of adults' correction, until children finally manage to memorize, one by one, all the irregular verb past tenses and override the more convenient rule for regular verbs. Irregular plurals and comparators are a similar source of confusion, which is why you may hear a preschooler describe a trip to the circus thus: The goodest part was those mans with the funny feets!

Emilie sits on her father's lap and excitedly stares at the shiny brass bell that the research assistant across the table is holding. Making sure Emilie is watching, the assistant places the bell into one of two matching wells in the table and then quickly covers both wells with identical cloths. Emilie is eager to grab the bell, as any eight-month-old would, but her father gently holds back her arms while the researcher distracts her with a funny face. After five seconds, Dad is signaled to release her arms, and Emilie uncovers the right-hand well and happily grabs the bell.

Now Emilie watches the researcher hide the bell in the other well, the one on the left. Again, Dad restrains her arms, both wells are covered, and her gaze is diverted to the experimenter's face. But this time, after another five-second delay, Emilie reaches back to the right-hand well and seems surprised not to find the brass bell in it.

Why didn't she reach to the left-hand well where she clearly saw the bell placed this time? The reason is simple; her frontal lobes aren't up to speed. Neither her working memory nor her inhibitory power are great enough to override the potent urge—or, more precisely, her procedural memory—to reach right back to the well where she had already successfully retrieved the bell.

This classic experiment was originally designed by Jean Piaget and is known as his "A not B" task, because virtually all babies Emilie's age reach for the toy correctly when it is first hidden in well "A," but err when the toy is moved to well "B." Though Piaget would attribute Emilie's mistake to the lack of "object permanence"—the memory that objects continue to exist even when you can't see them—we now know that he underestimated babies in this regard. Emilie remembers the bell's location in well B; she keeps her eyes there even as her hand is reaching to the wrong spot. Her problem is keeping this information in mind while simultaneously blocking her impulse to reach back to well A. She can do it if the interval between hiding and retrieval is short enough, no more than two or three seconds, but keeping track of everything—remembering that the bell is in well B, that she has to remove a cloth in order to retrieve it, and that she must not reach to well A—for five full seconds is simply more than she can manage.

"A not B" is harder than it seems; it requires planning, inhibition, working memory, and at least a minimal attention span—all frontal-lobe functions that are still rudimentary in babies Emilie's age. Each of these skills, however, will come to life over the next few months, as her frontal lobe kick; into action. By nine months, she can remember for as long as six seconds that the hiding place was switched, and by twelve months, she can remember it for a whopping ten seconds.

One of the more startling findings about early enrichment is the effect of music. You can hardly pick up a newspaper without seeing some kind of reference to how Mozart makes people smarter. The governor of Georgia recently proposed spending $105,000 of state money to provide every newborn baby with a compact disc of classical music, citing its positive effects on brain development and spatial and mathematical skills. What is it about classical music that is so good for mental function, and are children particularly susceptible to its magic?

Almost all the research on this subject has been performed by one group of neuroscientists from the University of California at Irvine. They were struck by the observation that people who are musically talented are often also talented at skills involving spatial-temporal integration, such as mathematics, chess, and engineering. Perhaps, they argued, music directly activates the same patterns of spatial-temporal activity in the brain areas involved in these forms of reasoning. Of course, music itself has no spatial component, but recall that pitch is converted into a spatial map by the inner ear. Our brains, then, experience music as simultaneous patterns in both space and time, perhaps not unlike the kind of mental patterning required to plot a chess strategy, a geometry proof, or a building's construction. According to this view, certain types of music should be better than others at promoting spatial-temporal reasoning, which is the hypothesis these researchers set out to test.

They began with a bunch of willing college students. One group of undergraduates spent ten minutes listening to a Mozart piano sonata, a second group listened to a relaxation tape, and a third group sat in silence for the ten minutes. Immediately afterward, all three groups were tested using a series of spatial reasoning tasks, such as figuring out the pattern in a series of figures, or what a piece of paper would look like after going through a sequence of mental folding and cutting. The results were quite striking; the Mozart group scored some nine points higher in spatial IQ than the relaxation and silence groups. In a follow-up study, the researchers added the repetitive, minimalist music of Philip Glass to the comparison. Again, only the group who listened to Mozart showed significant improvement on the folding-and-cutting task, and none of the groups differed on a test of short-term memory. So it does look as though certain fairly complex types of music do specifically enhance spatial-temporal reasoning, perhaps by exercising optimal patterns of neural activity in the right hemisphere.

Responsiveness is closely related to nurturing. For infants, responsive caregiving means not only prompt responding to a baby's physical needs interaction. Babies do cry out of boredom and their verbalizatinn—all that interaction. Babies do cry out of boredom, and their verbalization—all that enchanting cooing and babbling—is not just idle practice. They want and expect you to reply, to engage them in "protoconversation," and to light up their day with your interesting facial expressions, their innately preferred stimulus. Verbal responsiveness is essential to language development, but it also critically shapes children's emotional reactions and self-awareness. No matter what a child's age, responsive parenting means really listening to your child, taking the time to understand what he or she is trying to say, and engaging in lots of verbal give-and-take. Sensitive, responsive caregivers also appreciate that every child is different. They respect each child's individual needs and, equally important, teach that parent's own needs should be respected in return.

Involvement is another obvious feature of good parenting, but it may not always be clear what the best way is to go about it. "Involved parenting" doesn't mean driving your child around to lessons or arranging play dates where you sit and talk with other adults. It means direct, one-on-one interactions, in which all of your attention is focused on a joint activity with your child—reading a story, making up a song, building a sand castle, taking a nature walk, helping with homework. Several studies have found a relationship between children's IQ or academic achievement and the amount of time they spend in shared activities with their parents, also known by the familiar phrase "quality time."

What kind of shared activities are best for promoting children's cognitive development? Not, as some parents think, those involving a lot of academic instruction. Parents don't need to drill their preschoolers in phonics or hold flash-card sessions with their young infants to maximize their intellectual potential. Although some kids can benefit, in the short run, from early tutoring in reading or arithmetic, what works best in the long run are measures that foster children's enthusiasm, industry, perseverance, and motivation to learn. For babies, this means playful interactions that focus their interest on specific objects, concepts, and feelings. Recall that babies whose mothers (and presumably fathers) do a better job of encouraging their attention actually end up smarter than those whose mothers make less effort in this regard. Their vocabularies grow faster, they are more exploratory, and they even score higher on IQ tests as early as age four and as late as eighteen years. The best way to sustain babies' interest, given their rapid habituation, is to present variations on a theme—move their arms in different ways, or focus on different body parts, colors, shapes, or sounds. Encouraging children's attention, even very early in infancy, helps foster the persistence and motivation they need to master ever more difficult challenges.

For toddlers and preschoolers, choose something you enjoy—catching butterflies, putting a train set together, baking cookies, gardening, drawing on the computer, folding laundry, and of course, reading. Kids this age want to learn from their parents. They seek us out, following us around the house saying, "I see," and "I do it." By including them, you can teach that there's pleasure in accomplishment, experimentation, and creativity. Working side by side, parents can usually coax their children to do a little more, try a little harder, than they would on their own, giving them a sense of mastery that bolsters their confidence for future endeavors. Working together also gives children a close-up view of mature thinking in action, a model of how to observe, organize, and remember details, and also, ideally, of how to find joy in intellectual discovery.

In the case of a mother's more general nutritional status—her tota caloric intake—the brain is actually less sensitive during the first three to four months of gestation. In spite of its massive developmental changes, the fetus grows surprisingly little in size during this period, so its growth is not very dependent on the mother's diet. (This is probably no accident, since women are often unable to consume many calories because of first-trimester nausea.) Beginning around midway through gestation, however, and continuing until about two years after birth, the brain's growth is highly sensitive to the quantity and quality of nutrition it receives. This sensitive period coincides with the great spurt in synapse development, dendritic growth, and myelination, which together wire up the brain and also greatly increase its total weight. The quality o{ nutrition during this period has a profound impact on a child's future cognitive, emotional, and neurological functions.

Because this sensitive period begins before birth, it means that a mother's diet can shape her baby's brain development. And because it continues throughout infancy and toddlerhood, it means that special attention must be paid to a child's diet during these first two years. Nutritional deficits can be very specific, such as insufficient iodine, iron, or vitamin B12 intake, each of which can permanently alter brain and cognitive development if it continues for any substantial portion of the sensitive period. It is more common. however, for young children to suffer from a generalized nutritional deficiency—too few calories during gestation and early life—that can permanently compromise their brain development. Insufficient nutrition threatens the brain if it occurs at any time during the sensitive period, but is more devastating the earlier in this period it occurs and the longer it lasts, and when the lack of calories is compounded by inadequate protein intake.

The effects of malnutrition have been thoroughly studied in experimental animals, where we have achieved a fairly detailed understanding of the timing and type of nutrients needed for optimal brain development. Unfortunately, plenty of data are also available for human populations. A large proportion of children in the world are undernourished because of famine, poverty, war, and other natural or man-made disasters. It is through studies of such children that we have learned the ways in which inadequate early nutrition can permanently impair brain function. Children who were undemourished as fetuses or infants tend to score lower on IQ tests, perform more poorly in school, have slower language development, exhibit more behavioral problems, and even have difficulties with sensory Integration and fine motor skills, compared with children from the same culture who were adequately nourished. The earlier the malnourishment begins (starting with midpregnancy) and the longer it lasts, the greater will be the resulting problems and the less likely they can be overcome later on. By comparison, adults who undergo even the most extreme starvation do not suffer any intellectual impairment. Thus the brain has a special sensitive period for nutrition in infancy corresponding to the phase of massive synapse growth and axon myelination, both of which require considerable metabolic energy.

Babies of malnourished mothers are small at birth, with correspondingly smaller head sizes than babies well nourished in the womb. Within the normal range, birth weight and head size are only modestly related to later intelligence. But babies in the lowest tenth percentile, whose birth weight is less than four and a half pounds, do have a higher incidence of neurological impairment and mental deficits than larger infants. And malnourished babies are very likely to be in this smallest group.

Birth weight, unlike many traits, is influenced much more by a mother's nutrition than by heredity. Optimally, a pregnant woman should gain about 20 percent of her ideal prepregnancy weight (for example, twenty-six pounds for a 130-pound woman). Bigger is generally better, but there is a limit. (See Figure 17.3.) Babies who are very large at birth are likelier to cause a difficult delivery, and the brain is the organ most vulnerable to damage during a complicated birth. For optimal development a woman needs to consume about 300 extra calories per day during pregnancy, and 500 to 600 extra calories during lactation. It is recommended that many of these additional calories come from protein, which is especially important for brain development; women are advised to consume an extra lo to 12 grams of protein per day during pregnancy and 12 to 15 grams during lactation.

It just so happens that motherese is in many ways ideally suited to stimulate young babies' sense of hearing. Its unhurried cadence is easier for babies to follow, since as we've seen, their nervous systems process auditory information at least twice as slowly as adults. Its louder, more direct style helps babies distinguish it from background sounds and overcomes the fact that their hearing is much less sensitive than adults'. Its simpler words and highly intonated structure—with wide swings in pitch and loudness that enhance the contrast between sequential syllables—make it much easier for babies to distinguish individual parts of speech. And finally, its high pitch corresponds to babies' most sensitive frequency range from the age of about three months onward. In many ways, then, motherese is an optimal auditory stimulus for babies, especially after the immediate newborn period, and it is particularly good for them as they begin to acquire the basics of their native language.

Mothers are not the only ones who speak in "motherese." Fathers, older siblings, and others also tend to talk to infants and small children in this special "baby talk" that my colleague found so objectionable. The same speech pattern has also been observed among infant caregivers across many different cultures. It's hard to say whether we use this speech purely instinctively or because we learned it when we ourselves were children. But whatever the reason parents begin speaking to their babies in motherese, the reason they continue to do so is because the babies respond better to it than to normal speech. For instance, four-month-old babies given a choice between listening to recordings of a strange woman speaking in motherese or in regular adult speech preferred the former, judging by the number of times they turned their heads to activate each recording. The earliest that babies have been shown to be capable of recognizing motherese is about five weeks of age. when they will suck more to hear recordings of their mothers speaking in a highly inflected voice than in a flat monotone.

The preference for motherese probably begins forming in the womb, where the intonation and pitch of the mother's voice are transmitted more faithfully than her specific speech sounds. The preference is then soundly reinforced after birth, since this mode of speech is inevitably accompanied by lots of affection and attention. Given the emotional reinforcement and the auditory features that make it so optimally stimulating to their hearing, motherese is one of the most potent forms of stimulation a young baby receives.

The recent trend of putting young babies to sleep on their backs also appears to be having an effect on their motor skill acquisition. This posture, which has proven advantageous in reducing the number of SIDS fatalities, does not permit babies to exercise their arm and neck muscles as much as and see the world. In one recent study, pediatricians found that babies who slept on their backs were significantly slower to roll over, sit, crawl, and pull to stand than babies who slept on their stomachs. Fortunately, the delay was modest—still within the normal range for each milestone—and does not justify abandoning back-sleeping as the preferred posture for preventing SIDS. Nonetheless, parents should keep in mind the advantages of upper-body exercise in the early months and attempt to give their babies as much 'tummy time" as possible during their waking hours.

Nonetheless, our early touch experiences determine the extent of possible tactile sensitivity. They also play a surprisingly potent role in the overall quality of brain development. We have already seen in Chapter 2 how rats raised in a highly enriched environment develop a thicker cerebral cortex and are actually cleverer than rats raised in a standard laboratory environment. A good share of this enriching experience involves tactile sensation. When young rats are provided with new toys, they excitedly paw, nuzzle, and climb atop them, increasing the electrical activity and ultimately the size of their somatosensory cortexes. If the same toys are left in the cage for several days, the rats grow bored with them, and their cortexes begin to shrink back in size. But if the toys are changed at least twice a week, the increases in cortical size persist.

It is almost frightening, as a parent (though not as a toy manufacturer!). to contemplate the implications of these experiments. Touch experience is essential not only for the development of touch sensitivity but for general cognitive development as well. Fortunately, toys are not the only source of touch stimulation that can elicit these changes. Comparable effects on rats' brains and psychological performance occur when pups receive extra grooming by their mothers or handling by experimenters during the early weeks of life. So we don't necessarily have to break the bank on toys to provide young children with adequate stimulation. Anything that increases a baby's variety of touch stimulation is likely to enhance many aspects of brain and mental development.

Piaget had his own way of assessing brain maturation during this period, using his now-famous "conservation" tasks, try this one out on your tour-to-eight-year-old: fill two identical short, squat glasses with equal volumes of water, and ask your child, "Do the two glasses contain the same amount of water, or does one have more?" Now, pour all the water from one of these 'lasses into a tall, narrow glass, and ask your child the same question.

Four-year-olds almost invariably say that the tall glass has more water in it; the difference in the level of the water is simply too great for them to believe that the amount could be the same, even if you pour it back into the smaller glass and show them that it hasn't changed. Eight-year-olds, by contrast, know that the amount cannot have changed. If you press them on why the two look so different, they will tell you that the difference in height is made up for by width, and they will probably even pour the water from the taller glass back into the short one to prove their point.

The difference between four and eight years is the birth of reason, when children finally begin to trust their own thought processes, even over what their senses may be telling them. Piaget called it the emergence of "operational" thinking, when children actually begin applying logic to solve problems. Though younger children can often add a few numbers or recognize some written words, it is only in the latter half of this period—from six on— that most of them click into the rules of thought and realize, for instance, that addition is the opposite of subtraction or that letter sounds flow together into words. Drill them though you may, most four- and young five-year-olds simply can't make these kinds of conceptual connections.

Brain wiring begins with the outgrowth of axons. Once a newborn neuron has migrated, planting its cell body in a permanent position, it sends out a fine axon shoot with an enlarged tip known as a growth cone. At the end of the growth cone are about a dozen long tentacles that shoot out in all directions and act like radar, picking up all manner of navigational signals. They feel out the best-textured surfaces, sniff around for chemical cues, and even use tiny electrical fields to help the axon find its way to appropriate targets. Axons can grow to very great lengths, so long-distance connections, which pose the greatest challenge, tend to get an early start, at a time when the absolute distance between any two parts of the embryo (say, the spinal cord and the toe) is still comparatively short. Axon guidance also makes use of specific chemical attractants, released, much like insect pheromones, by potential target neurons to attract synaptic mates over relatively long distances. Led by their own genetically coded receptor molecules, these axons can't help but elongate in the direction of an ever-increasing concentration of the attractant molecule until they reach its source, the target neurons with a matching chemical identity.

Once an axon completes its traverse, whether near or far, it branches out extensively, contacting up to hundreds of target neurons that have released the same potent lure. Contact leads to synapse formation, but these initial connections are promiscuous: both far too numerous and highly unselective. During infancy and early childhood, the cerebral cortex actually overproduces synapses, about twice as many as it will eventually need. The initial wiring scheme is thus quite diffuse, with a lot of overlap that makes information transfer both imprecise and inefficient. It's as if all those billions of phones were first connected as party lines; you could dial Grandma at any of thousands of numbers, but it's unlikely she'd be the first to answer.

Why does the brain bother to produce so many excess synapses? Why not save time and energy and simply wire things up precisely from the start? The answers to these questions cut right to the core of the nature/nurture issue.

Up to now, genes have been largely responsible for establishing brain wiring. They prescribe all the early targeting cues—the pheromones that attract one class of axon to a particular class of neuron, the surface receptors that sense these attractants (or in some cases, repellents), as well as the receptors for other chemical, textural, and electrical cues that guide axon growth and synapse formation. But the fact is that there are not nearly enough genes in the entire human genome to accurately specify every one of our quadrillion synapses. There are perhaps 80,000 genes scattered among the miles of DNA in our chromosomes, and even if a generous half of these were allotted to the delicate job of brain wiring (after all, the body does have some other important functions to perform with its genes), we would still be far short of having enough cues to specify an accurate wiring diagram for the entire brain.

This is where "nurture" steps in and finishes the job. By overproducing synapses, the brain forces them to compete, and just as in evolution or the free market, competition allows for selection of the "fittest" or most useful synapses. In neural development, usefulness is defined in terms of electrical activity. Synapses that are highly active—that receive more electrical impulses and release greater amounts of neurotransmitter—more effectively stimulate their postsynaptic targets. This heightened electrical activity triggers molecular changes that stabilize the synapse, essentially cementing it in place. Less active synapses, by contrast, do not evoke enough electrical activity to stabilize themselves and so eventually regress. (See Figure 2.7.) It's "use it or lose it" right from the start; like other forms of Darwinian selection, this synaptic pruning is an extremely efficient way of adapting each organism's neural circuits to the exact demands imposed by its environment.

Our best evidence for how experience guides synaptic selection comes from studies of visual development... But there's another dramatic demonstration—some classic experiments on laboratory rats that were inspired by something Charles Darwin himself described back in 1868.

Ever the careful observer, Darwin rounded up a bunch of rabbits, measured their head and body sizes, and found that those raised in captivity had far smaller brains, relative to body weight, than those that grew up in the wild. Compared to the wild rabbits, Darwin realized, the domestic rabbits "cannot have exerted their intellect, instincts, senses and voluntary movements, either in escaping from various dangers or in searching for food," so that "their brains will have been feebly exercised, and consequently have suffered in development."

A century later, neurobiologists finally started to figure out how a challenging environment stimulates brain growth. Much like Darwin's rabbits. laboratory rats that have been reared in an "enriched" environment—in a large cage containing several litters and a wide variety of "toys" to see. smell, and manipulate—^have larger brains, with a notably thicker cerebral cortex, than those raised in an "impoverished" environment—isolated, in a small empty cage, without any social stimulation and a bare minimum of sensory experience. The reason their cerebral cortex is bigger, researchers have found, is that their neurons are larger, with bigger cell bodies, more dendritic branches, more spines, and more synapses than those in the brains of impoverished rats. In other words, the extra sensory and social stimulation actually enhances the connectivity of the enriched rats' brains, a difference that probably explains why they are also smarter—they learn their way around a baited maze significantly faster—than their impoverished laboratory mates.

It is no great stretch to see the implication of these experiments for human development: A young child's environment directly and permanently influences the structure and eventual function of his or her brain. Everything a child sees, touches, hears, feels, tastes, thinks, and so on translates into electrical activity in just a subset of his or her synapses, tipping the balance for long-term survival in their favor. On the other hand, synapses that are rarely activated—whether because of languages never heard, music never made, sports never played, mountains never seen, love never felt—will wither and die. Lacking adequate electrical activity, they lose the race, and the circuits they were trying to establish—for flawless Russian, perfect pitch. an exquisite backhand, a deep reverence for nature, healthy self-esteem— never come to be.

The magnitude of this synaptic sorting is enormous. Children lose on the order of 20 billion synapses per day between early childhood and adolescence. While this may sound harsh, it is generally a very good thing. The elimination of stray synapses and the strengthening of survivors is what makes our mental processes more streamlined and coherent as we mature; the party lines sort themselves out into clear, private, efficient channels for information transfer. On the other hand, it may also explain why our mental processes become less flexible and creative as we mature. Although the brain continues to exhibit certain more subtle forms of plasticity in adulthood (which is, after all, the way we learn or remember anything at all), it is never as malleable as in childhood.

First of all, language stimulation should begin very early: by just three years of age, children are already headed down vastly different paths of verbal achievement as a result of their cumulative experience with language. Ideally, language stimulation should begin at birth, since we know that newborns' brains are already attuned to human speech and immediately start learning the sounds of their mother tongue. In fact. Fowler's group found that babies who entered their program between six and eight months of age were not as successful as those who began at the earliest age, three months, so clearly earlier is better.

Secondly, the quantity of language is critical: the more words a child hears, the larger her vocabulary will be, and the faster it will continue to grow. But it cannot be overemphasized that this quantity means the number of words addressed to the child. Mothers aren't doing their kids any favors by talking on the phone all day; day-care workers don't help by conversing only with other workers; nor is television an adequate way to increase young children's language exposure. (Indeed, at one point deaf parents were advised to leave the TV on for their hearing babies, but it never succeeded in teaching them spoken language.) A baby can begin to make sense of language only when it refers to something she can directly relate to. Parents and other caregivers should thus talk frequently to their babies and try, whenever possible, to focus on the here and now: pointing out and labeling the objects, people. and events in their immediate environment, especially the babies' own actions, feelings, and attempts at speech.

Which brings us to the quality of language to which a child is exposed. Language addressed to young children needs to be simple, clear, and positive in tone in order to be of maximum value. Fortunately, most caregivers already use a special style when speaking to infants and young children. As noted in Chapter lo, babies clearly prefer the higher pitch, highly intonated style, and slower pace of "motherese," and recent evidence suggests that it even helps them in the earliest stages of phoneme-learning. But it's important to avoid the kind of muddled baby-talk that tums a sentence like "Is she the cutest lit¬ tie baby in the world?" into "Uz see da cooest wiwo baby inna wowud?" Care¬ givers should try to enunciate clearly when speaking to babies and young children, giving them the cleanest, simplest model of speech possible.

Of course, it's easy to say that speech should be at a level your child can understand, but it's not always easy to figure out what that level is. For instance, older babies understand much more than they can say, so you need not limit your speech to single syllables or words. On the other hand, there's evidence that even Sesame Street does more harm than good for children under eighteen months, probably because it comes at the expense of more direct, positive parental interaction. (But it is great for preschoolers.) At every age, parents need to seek out that happy medium of speaking to their child in a way that is largely within his reach of understanding but also stretches him just a bit beyond it.

Remarkably enough, the most obvious influence over children's language development turned out to be the mere amount of parents' talking; children whose parents addressed or responded to them more in early life had larger, faster-growing vocabularies and scored higher on IQ tests than children whose parents spoke fewer words to them overall. Parents who talk more inevitably expose their children to a greater variety of words and sentences, so a correlation also turned up between the diversity of parents' language—the number of different nouns and adjectives they used, and the length of their phrases and sentences—and their children's linguistic progress.

In addition to these quantitative features, Hart and Risley discovered a particular qualitative aspect of parental language that seems to especially influence children's language: the amount of positive versus negative feedback children hear. Youngsters who heard a larger proportion of no, don't, stop it, and similar prohibitions had poorer language skills than three-yearolds who had received less negative feedback. Of course, no parent of toddler-aged children can avoid all prohibitions, but those who kept their negative responses to a minimum, emphasizing instead positive responses, such as repeating their children's vocalizations or following them with ques¬ tions or affirmations, fostered better language development.

A follow-up study on the same group of children reveals that these differences in verbal skills persisted well into the grade-school years; by third grade, children whose parents spoke more to them during the first three years continued to excel at various language skills, including reading, spelling, speaking, and listening abilities. So even after children enter school, when their parents cease to be the sole influence over their cognitive development. their early language exposure has created a lasting legacy in their language achievement.

There is another, very disturbing side to Hart and Risley's report. In selecting the forty families tor their study, they deliberately chose a crosssection of American socioeconomic classes. When the researchers factored in these differences, it became blatantly clear that virtually every feature of parenting style improved substantially as families ascended the ladder of educational and financial advantage. Even something as simple as the number of words addressed to young children tended to increase dramatically, with chi dren on welfare hearing an average of 600 words per hour addressed to them, as compared with 1,200 for children of working-class families and 2,100 for children with professional parents. Socioeconomic level also correlated strongly with the type of feedback parents tended to give their children. On average, professional parents were heard to praise or otherwise respond positively to their children seven times more often than welfare parents, and they doled out negative feedback—those particularly toxic prohibitions and imperatives—only half as frequently. With such enormous differences in both the quantity and quality of interaction with their parents, it's not hard to see how children from different socioeconomic groups are propelled onto wildly different trajectories of language-learning.

The social and political implications of these findings are staggering. Obviously, it would take a massive effort to overcome these extreme differ¬ ences in children's early language experience. But it's important to realize that socioeconomic class per se is not the primary factor determining chil¬ dren's language achievement. For while children's fate may seem to be sealed by their level of economic advantage, what really matters is their parents' style of interacting with them. In other words, if we look just within a single socioeconomic group, like the twenty-three families that made up the "working-class" rank in Hart and Risley's study, parenting style turns out to be a much better predictor of each child's language skills than the parents' precise financial and educational attainment. Within this group, parents who talked more to their children, who used a greater variety of words and sentences, who asked rather than told their children what to do, and who consistently responded in positive rather than negative ways to their chil¬ dren's speech and behavior, tended to raise more verbally gifted children than those who were poorer at these parenting skills. Similar findings have been reported in a study of professional-class children in Chicago: those whose mothers addressed more words to them in the second year of life had the fastest-growing vocabularies, ou even m higher socioeconomic ranks, there is enough variety in parenting styles to significantly affect the quality of children's language development, exploding, as some call it, "the myth of the educated parent."

There are two main reasons for concern about exercising during pregnancy. One is that it may reduce the baby's oxygen supply, since exercise, like other sources of stress, reduces blood flow to the uterus. Another risk is overheating. As we have already seen, fetal development is highly sensitive to temperature, and elevations of more than 2^0 C (or above 1020F) can increase the risk of miscarriage and affect the formation of the brain and eyes.

Despite these theoretical concerns, there is little evidence that mothers who exercise or are physically very active have any particular problems with their pregnancies. Most studies have found no difference in prematurity or Apgar scores (measures of newborn health taken one and five minutes after birth) between babies born to mothers who exercise and those who are more sedentary. With regard to birth weight, there are conflicting reports about the effects of exercise; some studies have found that women who exercise have significantly smaller babies, but several recent studies refute these findings, and one actually found that the more women exercised, the larger their babies tended to be. The key factor here appears to be the amount of weight the mother gains. If exercise prevents the mother from gaining adequately. she is likelier to give birth to a low-weight baby, but women who exercise and gain sufficient weight do not appear to compromise their baby's brain and bodily development.

Offsetting its potential harm are the numerous benefits of exercise, many of which can be traced to the fact that it elevates a mother's levels of betaendorphin—-a morphine-like substance produced by the body that blocks the transmission of painful stimuli to the brain. In addition, exercise actually lowers the level of another stress hormone, Cortisol, in pregnant women. These hormonal changes explain why exercise often counteracts the emotional impact of other sources of stress. Exercise generally increases a woman's sense of well-being, and based on what we know about anxiety and stress, this is likely to have a positive influence on the fetus.

The best-documented benefit of exercise comes in labor and delivery. Women who exercise regularly fare much better during childbirth compared with women who do not. They perceive it to be less painful, and indeed it may be; one study found that women who exercise spend just twenty-seven minutes in the second stage of labor—pushing—compared with fifty-nine minutes for women who did not exercise during pregnancy. Shorter labor is generally beneficial to the baby, since it reduces the risk of complications. including oxygen deprivation of the brain.

Doctors have traditionally been rather conservative about exercise during pregnancy, but current evidence indicates that it is safe for most women, especially those who were already physically fit before conceiving. Exercise should be kept to a "moderate" level, meaning that it does not elevate the woman's heart rate above 70 percent of its maximum rate (220 beats per minute minus one's age in years)—for example, 133 beats per minute in a thirty-year-old. Because there is evidence that a woman's oxygen reserves are lower in the third trimester, it is a good idea to scale down exercise, particularly weight-bearing types, toward the end of pregnancy, as most women are lined to do anyway. Other situations to avoid include: (1) exercising at high altitudes (more than 10,000 feet), because the placenta is already having to compensate for lower oxygen levels; (2) exercising in hot weather. because of the risk of overheating the fetus; and (3) scuba and snorkel diving, because of the potential risk of accumulating excess nitrogen and other gases in fetal tissues. But other water immersion sports, like swimming and "aqua-jogging," are among the best forms of exercise for pregnant women. because the water helps dissipate excess heat from the mother's body.

But with the right balance, parents can modify even the most difficult side of their children's temperaments. As an example, consider those 15 percent or so of toddlers who are very inhibited—kids like Andrew, whose right frontal lobe explodes with anxiety whenever he's confronted by new people or a new environment. While many of these children don't change, about 40 percent do lose their extreme timidity by kindergarten. Researchers have observed that these are the youngsters whose parents, though sensitive, manage to gently challenge them, encouraging them to face their fears and learn how to cope with minor stresses, thereby coaxing along those connections on the left side of the brain.

Patricia is one parent who's decided to try challenging her child more. She's enrolled Andrew in nursery school, now encourages him to be more adventuresome on the playground, and has begun traveling with him, so he can spend his first nights away from home. So far he's not too happy about it, but he is showing signs of gradually adapting, rising to the challenge his parents are laying out for him. Before long he'll probably actually enjoy school and will undoubtedly even make a few friends. Though it's likely that he will always be a pretty cautious kid, he'll surely have a happier youth than it his parents hadn't stepped in and deliberately helped rewire his limbic system.

The perfect parent, if she (or he) existed, would devote herself full time to the care and teaching of her child. She would begin, even before conception, by shoring up her folic acid reserves and purging her body of any chemical remotely suspect. Once pregnant, she would never touch a drop of alcohol, pump her own gasoline, get less than eight hours sleep, or allow herself to be stressed in any way. She would have an ideal, unmedicated, and uncomplicated delivery, and breastfeed from the moment of birth until the child was potty-trained. She would know precisely how to stimulate her baby, but also how to avoid over-stimulation. She would spend hours every day playing with him—singing, cuddling, talking, massaging, exercising, reading, showing him how all kinds of toys and other fascinating objects work—and never have to leave him in his swing for half an hour while trying to make supper or balance the checkbook. Her house would be perfectly baby-proofed, so he could explore every comer and rarely hear "No!" She'd take him on all kinds of different outings, always giving him her full attention, and never grow annoyed when he pulled all the vitamins off the shelf at the pharmacy or whined for cookies at the grocery store. She'd introduce him to other children, all with similarly perfect parents, and gladly clean up after the messiest play dates. She'd start him on piano/tennis/dance/French/swimming/art/violin/computer/Spanish/tumbling lessons at age three (practicing herself, to provide a good role model) but, if he showed no interest, would happily forfeit the ten weeks' tuition. She'd send him to the perfect preschool, using their time apart to brush up on the latest child-rearing information and prepare all sorts of new and interesting educational activities for him. And of course, she wouldn't do it alone. She'd have the "perfect spouse" right alongside, equally loving/stimulating/nurturing/teaching their child every step of the way.

There may actually be one or two parents in the world like this. And perhaps their kids will turn out to be the most brilliant, talented people ever. Then again, you have to wonder what children learn from parents whose only focus in life is their offspring. The fact is that children pick up much more than mere cognitive skills from their parents and other caregivers. They also learn how to work, share, love, nurture, juggle, and enjoy life. Once again, it is the model we set, rather than the specific teaching we attempt, that is going to have the biggest impact on a child's cognitive abilities and success in life.