The History of Evolution of Organisms consists of two kindred and closely connected parts: Ontogeny, which is the history of the evolution of individual organisms, and Phylogeny, which is the history of the evolution of organic tribes. Ontogency is a brief and rapid recapitulation of Phylogeny, dependent on the physiological functions of Heredity (reproduction) and Adaptation (nutrition). The individual organism reproduces in the rapid and short course of its own evolution the most important of the changes in form through which its ancestors, according to laws of Heredity and Adaptation, have passed in the slow and long course of their palaeontological evolution.
Kohlberg outlined a progressive process for moral development:
1. Avoiding punishment. Moral reasoning starts out at a fairly primitive level, focused mostly on avoiding punishment. Kohlberg calls this stage pre-conventional moral reasoning.
2. Considering consequences. As a child’s mind develops, she begins to consider the social consequences of her behaviors and starts to modify them accordingly. Kohlberg terms this conventional moral reasoning.
3. Acting on principle. Eventually, the child begins to base her behavioral choices on well-thought-out, objective moral principles, not just on avoidance of punishment or peer acceptance. Kohlberg calls this coveted stage post-conventional moral reasoning. One could argue that the goal of any parent is to land here.
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.
As they hear us talk, babies are busily grouping the sounds they hear into the right categories, the categories their particular language uses. By one year of age, babies' speech categories begin to resemble those of the adults in their culture. Pat conducted some even more complicated experiments with Swedish babies using simple vowels to see how early they start organizing the sounds of their language in an adult-like way. She showed that at six months the process has already begun. The six- to twelve-month time span appears to be the critical time for sound organization.
What might be happening to the babies between six and twelve months? One way of thinking about it is in terms of what Pat calls prototypical sounds. After listening to many r sounds in English, for example, babies develop an abstract representation of r—a prototypical r—that is stored in memory. When we want to identify a new sound, we seem to do it by unconsciously comparing the new sound to all of the prototypes stored for our language and picking the one that's the best overall fit. Once we've unconsciously done this, we distort the way we hear a sound to make it more like the prototype stored in memory than like the sound that actually hits our ears.
It's similar to what happens when you show people a drawing of something they've seen very often, a house, for example, and then ask them to copy it from memory. If the house you show them doesn't have a chimney, many people will add one to their drawing anyway, even though it wasn't in the original drawing they saw. Once they coded the picture as a house, they distorted their memory of it to make it more like what they think of as the prototypical house. We can do complicated analyses to show just what the prototypes of our speech sounds are and just how we distort what we hear to suit them. Our language prototypes "filter" sound uniquely for our language, making us unable to hear some of the distinctions of other languages. Pat's tests suggest that babies' language prototypes begin to be formed between six and twelve months of age.
It isn't just that younger babies have a skill they lose later on. Rather, the whole structure of the way babies organize sounds changes in the first few months of life. Before they are a year old, babies have begun to organize the chaotic world of sound into a complicated but coherent structure that is unique to their particular language. We used to think that babies learned words first and that words helped them sort out which sounds were critical to their language. But this research turned the argument around. Babies master the sounds of their language first, and that makes the words easier to learn.
When babies are around a year old, they move from sounds to words. Words are embedded in the constant stream of sounds we hear, and it is actually difficult to find them. One problem computers haven't yet solved is how to identify the items that are words without knowing ahead of time what they are. Try to find the words in a string of letters like theredonateakettleoftenchips. The string contains many different words: The red on a teakettle often chips or There, Don ate a kettle of ten chips and so on. Of course, in written language there are normally spaces between words. But in spoken language there aren't actually any pauses between words. That's why foreign speech sounds so fast and continuous, and that makes the Language problem very hard for computers to solve.
What are the babies' representations and rules like?
First, the babies' representations are rich and complex. As we've seen, they include ideas about how their face resembles the faces of others, how objects move, and how the sounds of a language are divided. The young babies' world is not simple. Babies translate the input at their eyes and ears into a world full of people with animated, expressive faces and captivating. intricate, rhythmic voices. It's also a world full of objects with complex multidimensional structure that move in a dizzying variety of ways.
Babies' representations are also abstract. They go beyond the data of immediate sensation. Most obviously these early representations link information from different senses: they link the way the tongue feels and the way it looks, the bounce of a ball and the boing sound it makes, the look of an open mouth and the sound of an aah.
But the representations go beyond sensation in other, more profound ways. They turn facial expressions into emotions. They convert two-dimensional images into three-dimensional objects. They take a continuous stream of noise and divide it up into discrete speech sounds. Even newborn babies end up with representations that are radically different from the input at their eyes and ears. The babies' world isn't concrete any more than it's simple. Babies already see the soul beneath the skin and hear the feeling behind the words.
These representations and rules lead young babies to interpret what happens to them in particular ways—to pay attention to some things and ignore others. At first they are particularly captivated by faces and voices; within a few days they pay special attention to familiar faces and voices. At first they pay special attention to the way things move and less attention to their shape or color or texture; later they will start to pay more attention to these properties of objects. At first and not others; later they will no longer attend to sound changes that once intrigued them.
Finally, the babies' representations and rules allow babies to form expectations, and even to make predictions, about new things that will happen in the future. When the babies' program gets information about a current event, it can generate a representation of a future event. When young babies see a toy car go behind the screen, they look ahead to the far edge babies flirt, they expect that their coos will be answered by adult goos. When they see an open mouth, they expect that they will hear an aah sound. They react in characteristic ways when their predictions turn out to be wrong and their expectations are dashed. They show conflict when the toy car doesn't appear to behave as it should, and they are distressed when their flirtatious advances are met with an impassive stony face. Just as the babies' world isn't simple or concrete, it also isn't limited to the here and now. Even very young babies can remember what happened in the past and predict what will happen in the future.
The significance of this inborn program goes beyond just the simple fact that there is a lot there to begin with. The baffling problem for philosophers and psychologists was always how we get from the raw, undigested matter of sensation—the "blooming, buzzing confusion"—to an understanding of the world. How do we even know which kinds of sensations to pay attention to? The answer the babies give is that we are never dealing with raw matter. There never is a blooming, buzzing confusion. From the very beginning we can understand the world, pick and choose what's important. know what to expect. From the time we're born, we run a program that translates the light and sound waves into people, objects, and language.
Babies start out believing that there are profound similarities between their own mind and the minds of others. That belief gives them a jump start in solving the Other Minds problem. But during the first three years they also observe the differences in what people do and say. Those differences stem from the fact that all minds aren't actually entirely alike. Babies and young children watch and listen with careful focused interest as their mother refuses to let them touch the lamp cord or as their older brother tells them they are completely wrong. This new evidence makes babies revise the beliefs they started out with.
Similarly, babies start out knowing that space is threedimensional and that objects move in predictable ways. They even reach out to objects and shrink away from them. By the time they are eighteen months old, as they watch and manipulate the things around them, as they play peekaboo and sort things into piles, they see those objects act in new ways and they look for ways to explain what they see. They learn that three-dimensional moving objects continue to exist no matter how they appear or disappear, and they learn that all those objects belong in categories. By three or four they have transformed those first categories into biological species and "natural kinds," as they begin to understand that kittens become cats and that tigers have guts inside and rocks don't.
Finally, babies start out making all the possible distinctions between the sounds of languages. Like citizens of the world. mierican newborns can distinguish African Kikuyu sounds as well as English sounds. By twelve months, as they repeatedly hear the sounds of their own language, babies create new representations that reflect the sound categories of their particular language. One-year-old American babies can't discriminate Kikuyu categories anymore, but they can discriminate the English categories better, and they have even become "English-sounding" babblers.
In each case the things babies already think influence where they will go next. They determine which events will engage them, which problems they will tackle, which experiments they will do, even which words they will listen to. Then babies change what they think in the light of what they learn.
Babies have another ability that man-made computers lack. They can do things. They can actively intervene in the world as well as passively learn about it. A one-year-old can reach for a new rubber duck, put it in his mouth, bang it against the side of the tub, splash it in the water, and watch his father's reactions to all of this. A key aspect of our developmental picture is that babies are actively engaged in looking for patterns in what is going on around them, in testing hypotheses, and in seeking explanations. They aren't just amorphous blobs that are stamped by evolution or shaped by their environment or molded by adults.
In Chapter Three we described how children need to figure out what's going on around them—they have a kind of explanatory drive. This drive pushes them to act in ways that will get them the information they need; it leads them to explore and experiment. The apparently pointless activities we call play often seem to be the result of this drive. Babies who are who are figuring out how we see objects play hide-and-seek; babies who are figuring out the sounds of language babble. It's all very serious fun.
The baby computers start out with a specific program for translating the input they get into accurate representations of the world and then into predictions and actions. But the interesting thing about these computers is that they don't stop there. Instead, they reprogram themselves. They actively intervene in the world to gather more input and check their predictions against that new input. The things they find out lead them to construct new and quite different representations and new and quite different rules for getting from inputs to representations. If we wanted to make a new computer as powerful as the biological computers, this is what it would have to be like.
When a three-month-old, a one-year-old, and a four-year-old look at the same event, they seem to have very different thoughts about it. They seem to transform the light waves and sound waves into different representations, and they use different rules to manipulate those representations. Children don't have just a single, fixed program that gets from input to output. Instead, they seem to switch spontaneously from using one program to using another, more powerful program. That makes babies and children look very different from the computers we have now...
How can we explain these changes? One idea might be that the changes are simply a result of the fact that babies grow, the way caterpillars change into butterflies as they grow. or the way we develop breasts and beards as we grow and reach puberty. The changes might just involve a genetic blueprint that unfolds on a particular maturational timetable. The child's program for understanding false beliefs might appear when she's four the same way her breasts appear when she's twelve. After all, we don't think that the caterpillar learns how to be a butterfly. Similarly, we might not think that the child learns about false belief any more than she learns how to have breasts.
Another very different possibility is that we change our ideas about the world just by taking in more and more information about it. We simply accumulate more and more input. Then we associate some pieces of that input with other pieces. We hear the dinner bell and food comes, and after a while we link the bell with the food. We give a particular answer to the experimenter's question and we get praise, and after a while we try to give answers like that. Babies could end up linking particular inputs to each other and to particular outputs in this sort of specific, piecemeal way.
The tests show that babies' preferences have nothing to do with the actual words mothers use. Babies choose motherese (or "parentese" or "caretakerese") even when the speaker is talking in a foreign language so infants can't understand the words, or when the words have been filtered out using computer techniques and only the pitch of the voice remains. Apparently they choose motherese not just because it's how their mother talks but because they like the way it sounds. Motherese is a sort of comfort language; it's like aural macaroni and cheese. Even grown-ups like it. Pat's graduate students discovered that listening to the lab tapes of motherese in a foreign language was a wonderful therapy for end-of-term stress. The mother's voice is an acoustic hook for the babies. It captures babies' attention and focuses it on the person who is talking to them.
The elaborate techniques of computer voice analysis reveal exactly what it is we do when we talk to an infant. The pitch of our voice rises dramatically, sometimes by more than an octave; our intonation becomes very melodic and singsongy; and our speech slows down and has exaggerated, lengthened vowels.
Motherese is a universal language. People across all cul:ures do it when they talk to their infants, even though they usually aren't aware of doing it at all. When mothers listen to recordings of themselves producing motherese, the reaction is: That can't be me. I sound really stupid. Should I be doing that? But they do it intuitively, without conscious awareness.
Why do we do it? Do we produce motherese simply to get the babies' attention? (It certainly does that.) Do we do it just to convey affection and comfort? Or does motherese have a more focused purpose? It turns out that motherese is more than just a sweet siren song we use to draw our babies to us. Motherese seems to actually help babies solve the Language problem.
Motherese sentences are shorter and simpler than sentences directed at adults. Moreover, grown-ups speaking to babies often repeat the same thing over and over with slight variations. ("You are a pretty girl, aren't you? Aren't you a pretty girl? Pretty, pretty girl.") These characteristics of motherese may help children to figure out the words and grammar of their language.
But the clearest evidence that motherese helps babies learn comes from studies of the sounds of motherese. Recent studies show that the well-formed, elongated consonants and vowels of motherese are particularly clear examples of speech sounds. Mothers and other caregivers are teachers as well as lovers. Completely unconsciously they produce sounds more clearly and pronounce them more accurately when they talk to babies than when they talk to other adults. When mothers say the word bead to an adult, it's produced in a fraction of a second and it's a bit sloppy. But when mothers say that same word to their infants, it becomes beeeeeed, a well-produced, clearly articulated word. This makes it easier for infants to map the sounds we use in language.
Why do the speakers of different languages hear and produce sounds so differently? Ears and mouths are the same the world over. What differs is our brains. Exposure to a particular language has altered our brains and shaped our minds, so that we perceive sounds differently. This in turn leads speakers of different languages to produce sounds differently. When and how do babies start to do this? Do they start out listening like a computer, with no categorical distinctions? Or do they start out with the categorical distinctions of one particular language, say English or Japanese or Russian?
We can't ask babies directly whether they think two sounds are the same or different, but we can still find out. Very young babies can tell us what they hear by sucking on a special nipple connected to a computer. Instead of producing milk, sucking on this special device produces sounds from a loudspeaker. one sound for each hard suck. Babies love the sounds almost as much as they love milk: they may suck up to eighty times a minute to keep the sounds turned on. Eventually, though. they slow down; they get bored hearing the same thing over and over again. When the sound is changed, however, infants immediately perk up and suck very fast again to hear the new sound. That change in their sucking shows that they can hear a difference between the new sound and the sound they heard before. Using this technique we can do the same r and l experiment we just described with adults. We can use a speech synthesizer to present the babies with a slowly and continuously changing consonant sound. Then we can test the babies to see which sounds they think are the same and which sounds they think are different.
Scientists anticipated that these tests would show that very young babies initially can't hear the subtle differences between speech sounds and only slowly learn to distinguish those that are important in their particular language, such as r and l in English. In fact, the results were just the reverse. In the very first tests of American infants listening to English, babies one month old discriminated every English sound contrast we threw at them. Moreover, the babies demonstrated the categorical perception phenomenon. They thought all the r's were the same and different from all the l's, just as adult English speakers do.
But then shortly afterward speech scientists discovered something even more remarkable. Kikuyu babies in Africa and Spanish babies in Mexico were also excellent at discriminating American English sounds as well as the sounds of Kikuyu and Spanish, and American babies were just as good at discriminating Spanish sounds—much better than American adults. The sophisticated Japanese scientists who strained to hear the difference between rake and lake would not have had any trouble doing so when they were forty or fifty years younger. Very young babies discriminated the sounds not only of their own language but of every language, including languages they had never heard. Infants were as good at listening to American English as they were at listening to African Kikuyu, Russian, French, or Chinese regardless of the country they were raised in. Pat also discovered that babies, unlike computers, make these distinctions no matter who is talking—a man or a woman, a person with a high squeaky voice or one with a deep resonant voice.
So babies start out knowing much more about language than we would ever have thought. Newborn babies already go well beyond the actual physical sounds they hear, dividing them into more abstract categories. And they can make all the distinctions that are used in all the world's languages. Babies are "citizens of the world." Perhaps we grown-up scientists failed to predict this because our skills are so much more limited. Our citizen-of-the-world babies clearly outperform their culture-bound parents.
However, there is some surprising evidence that young babies are actually not particularly interested if a blue toy car goes in one edge of the screen and a yellow toy duck emerges at the far edge on the same trajectory! A grown-up would assume the duck that came out was brand-new and the other toy was still there behind the screen. But young babies seem content to think the toy somehow magically became a new kind of thing behind the screen. The particular kind of category-crossing magic trick in which the scarf turns into a dove wouldn't be surprising to them. Although young babies can discriminate between yellow and blue, and between the duck shape and car shape, they don't seem to rely on these features to determine which object this is. By the time babies are a year old, however, it is easy to show that across a wide range of situations they are surprised when the car turns into a duck, which suggests they have developed a new view of categorization.
Babies do other things that suggest they have a new view of categories. Alison and Andy gave babies a mixed-up bunch of objects: four different toy horses and four different pencils. Alison would put her hands palm up on the table and watch what the babies did with the objects. Nine- and ten-month-olds picked up the horses and pencils, played with them, and often put them in her hands, but they did so pretty much at random. But twelve-month-olds would sometimes pick all the objects of one group, all the horses or all the pencils, and put them in a hand or in a single pile on the table. By the time they were eighteen months old, babies would quite systematically and tidily sort the objects into two separate groups, carefully placing a horse in one hand and then a pencil in the other. In one experiment a particularly fastidious and precise little girl (there actually are fastidious eighteen-month-olds) noticed that one of the pencils had lost its point. She looked carefully at both hands and then reached for her mother's hand to make a separate spot for this peculiar and defective object.
By the time they are two or three years old, children already seem to have a deeper conception of what it means for an object to belong to a category. They can go beyond the superficial appearance of an object and comprehend something about its essential nature. And they begin to understand that knowing an object's category lets you predict specific new things about the object. For instance, you can tell three-year-olds some new fact about a particular object, you can point to a rhinoceros and say, "This rhinoceros has warm blood." If you then tell them that another animal is a rhinoceros, they will say that it has warm blood, too. But they won't extend their new discovery to a triceratops, which looks like a rhinoceros, if you describe it as a dinosaur.