In fact, some evolutionary biologists believe this is why morning sickness still persists in human pregnancies. Morning sickness, which can last the entire day (and, for some women, the entire pregnancy), makes a woman stick to a bland, boring diet—if she eats much at all. This avoidance strategy would have kept our maternal ancestors away from the natural toxins in exotic or spoiled foods in the wild, unregulated menu of the Pleistocene diet. The accompanying fatigue would keep women from engaging in physical activity risky enough to harm the baby. Researchers now think it could make the baby smarter, too.

It is my contention that man is just like an ibis or a swallow or a sparrow in several key respects. He lives in large colonies. Males compete with one another for places in a pecking order. Most males are monogamous. Polygamy is prevented by wives who resent sharing their husbands lest they also share his contributions to child rearing. Even though they could bring up the children unaided, the husband's paycheck is invaluable. But the ban on polygamous marriage does not prevent the males from seeking polygamous matings. Adultery is common. It is most common between high-ranking males and females of all ranks. To prevent it males try to guard their wives, are extremely violent toward their wives' lovers, and copulate with their wives frequently, not just when thev are fertile.

That is the life of the sparrow anthropomorphized. The life of man sparrowmorphized might read like this: The birds live anc 3reed in colonies called tribes or towns. Cocks compete with one another to amass resources and gain status within the colony; it is known as "business" and "politics." Cocks eagerly court hens, who resent sharing their males with other hens, but many cocks, especially senior ones, trade in their hens for younger ones or cuckold other cocks by having sex with their (willing) wives in private.

The point does not lie in the details of the sparrow's life. There are significant differences, including the fact that human beings tend to have a much more uneven distribution of domi¬ nance, power, and resources within the colony. But they still share the principal feature of all colonial birds: monogamy, or at least pair bonds, plus rife adultery rather than polygamy. The noble savage, far from living in contented sexual equanimity, was paranoid about becoming, and intent on making his neighbor into, a cuckold. Little wonder that human sex is first and foremost in all societies a private thing to be indulged in only in secret. The same is not true of bonobos, but it is true of many monogamous birds. One reason the high bastard rates of birds came as such a shock was that few naturalists had ever witnessed an adulterous affair between two birds—they do it in private.

t. Baker and Bellis discovered that the amount of sperm that is retained in a woman's vagina after sex varies according to whether she had an orgasm and when. It also depends on how long it was since she last had sex: The longer the period, the more sperm stays in, unless she has what the scientists call "a noncopulatory orgasm" in between.

So far none of this contained great surprises; these facts were unknown before Baker and Bellis did their work (which consisted of samples collected by selected couples and of a survey of four thousand people who replied to a questionnaire in a magazine), but they did not necessarily mean very much. But Baker and Bellis also did something rather brave. They asked their subjects about their extramarital affairs. They found that in faithful women about 55 percent of the orgasms were of the high-retention (that is, the most fertile) type. In unfaithful women, only 40 percent of the copulations with the partner were of this kind, but 70 percent of the copulations with the lover were of this fertile type. Moreover, whether deliberately or not, the unfaithful women were having sex with their lovers at times of the month when they were most fertile. These two effects combined meant that an unfaithful woman in their sample could have sex twice as often with her husband as with her lover but was still slightly more likely to conceive a child by the lover than the husband.

But when she examined the record of history, Betzig was stunned. Her simplistic prediction that power is used for sexual success was confirmed again and again. Only in the past few centuries in the West has it failed. Not only that, in most polygamous societies there were elaborate social mechanisms to ensure that a powerful polygamist left a polygamous heir.

The six independent "civilizations" of early history—BabyIon, Egypt, India, China, Aztec Mexico, and Inca Peru—were remarkable less for their civility than for their concentration of power. They were all ruled by men, one man at a time, whose power was arbitrary and absolute. These men were despots, meaning they could kill their subjects without fear of retribution. Without exception, that vast accumulation of power was always translated into prodigious sexual productivity. The Babylonian king Hammurabi had thousands of slave "wives" at his command. The Egyptian pharaoh Akhenaten procured 317 concubines and "droves" of consorts. The Aztec ruler Montezuma enjoyed 4.000 concubines. The Indian emperor Udayama preserved sixteen thousand consorts in apartments ringed by fire and guarded by eunuchs. The Chinese emperor Fei-ti had ten thousand women in his harem. The Inca Atahualpa, as we have seen, kept virgins on tap throughout the kingdom.

Not only did these six emperors, each typical of his predecessors and successors, have similarly large harems, but they employed similar techniques to fill and guard them. They recruited young (usually prepubertal) women, kept them in highly defensible and escape-proof forts, guarded them with eunuchs, pampered them, and expected them to breed the emperor's children. Measures to enhance the fertility of the harem were common. Wet nurses. who allow women to resume ovulation by cutting short their breast-feeding periods, date from at least the code of Hammurabi in the eighteenth century B.C.; they were sung about in Sumerian lullabies. The Tang Dynasty emperors of China kept careful records of dates of menstruation and conception in the harem so as to be sure to copulate only with the most fertile concubines. Chinese emperors were also taught to conserve their semen so as to keep up their quota of two women a day, and some even complained of their onerous sexual duties. These harems could hardly have been more carefully designed as breeding machines, dedicated to the spread of emperors' genes.

Let us erect the four commandments of mating system theory. First, if females do better by choosing monogamous and faithful males, monogamy will result—unless, second, men can coerce them. Third, if females do no worse by choosing already-mated males, polygamy will result—unless fourth already-mated females can prevent their males fro monogamy will result. The surprising conclusion of game theory is therefore that males, despite their active role in seduction, may be largely passive spectators at their marital fate.

Low was looking to explain why young women have fat on their breasts and buttocks more than on other parts of their bodies. The reason this requires explaining is that young women are different from other human beings in this respect. Older women, young girls, and men of all ages gain fat on their torsos and limbs much more evenly. If a woman of twenty or so gains weight, it largely takes the form of fat on the breasts and buttocks; her waist can remain remarkably narrow.

So much is undisputed fact. What follows is entirely conjecture, and it was a conjecture that caused Low a good deal of sometimes vicious (and mostly foolish) criticism when she published the idea in 1987

Twenty-year-old women are in their breeding prime; therefore, the unusual pattern of fat distribution might be expected to be connected with getting a mate or bearing children. Standard explanations concern the bearing of children; for example, fat is inconvenient if it competes for space about the waist with a fetus. Low's explanation concerns the attraction of mates and takes the form of a Red Queen race between males and females. A man looking for a wife is likely to be descended from men who found two things attractive (among many others): big breasts, for feeding his children, and wide hips, for bearing them. Death during infancy due to a mother's milk shortage would have been common before modern affluence—and still is in some parts of the world. Death of the mother and infant from a birth canal that was too narrow must also have been common. Birth complications are peculiarly frequent in humans for the obvious reason that the head size of a baby at birth has been increasing quickly in the past 5 million years. The only way birth canals kept pace (before Julius Caesar's mother was cut open) was through the selective death of narrow hipped women.

Grant, then, that early men may have preferred women with relatively wide hips and large breasts. That still does not explain the gaining of fat on breasts and hips; fat breasts do not produce more milk than lean ones, and fat hips are no farther apart than lean ones of the same bone structure. Low thinks women who gained fat in those places may have deceived men into thinking they had milkful breasts and wide hip bones. Men fell for it— because the cost of distinguishing fat from heavy breasts or of distinguishing fat from wide hips was just too great, and the opportunity to do so was lacking. Men have counterattacked, evolutionarily speaking, by "demanding" small waists as proof of the fact that there is little subcutaneous fat, but women have easily overcome this by keeping waists slim even while gaining fat elsewhere.

In 1966, George Williams exposed the logical flaw at the heart of the textbook explanation of sex. He showed how it required animals to ignore short-term self-interest in order to "further the survival and evolution of their species, a form of self restraint that could have evolved only under very peculiar circumstances. He was very unsure what to put in its place. But he noticed that sex and dispersal often seem to be linked. Thus, grass grows asexual runners to propagate locally but commits its sexually produced seeds to the wind to travel farther. Sexual aphids grow wings; asexual ones do not. The suggestion that immediately follows is that if your young are going to have to travel abroad, then it is better that they vary because abroad may not be like home.'^

[...]

Williams was especially intrigued by creatures such as aphids and monogonont rotifers, which have sex only once every few generations. Aphids multiply during the summer on a rosebush, and monogonont rotifers multiply in a street puddle. But when the summer comes to an end, the last generation of aphids or of monogonont rotifers is entirely sexual: It produces males and females that seek each other out, mate, and produce tough little young that spend the winter or the drought as hardened cysts awaiting the return of better conditions. To Williams this looked like the operation of his lottery While conditions were favorable and predictable, it paid to reproduce as fast as possible—asexually. When the little world came to an end and the next generation of aphid or rotifer faced the uncertainty of finding a new home or waited for the old one to reappear, then it paid to produce a variety of different young in the hope that one would prove ideal.

Williams contrasted the "; "aphid-rotifer model" with two others: the strawberry-coral model and the elm-oyster model. Strawberry plants and the animals that build coral reefs sit in the same place all their lives, but they send out runners or corora branches so that the individual and its clones gradually spread over the surrounding space. However, when they want to send their young much farther away in search of a new, pristine habitat, the strawberries produce sexual seeds and the corals produce sexual larvae called "planulae." The seeds are carried away by birds; the planulae drift for many days on the ocean currents. To Williams, this looked like a spatial version of the lottery: Those who travel farthest are most likely to encounter different conditions, so it is best that they vary in the hope that one or two of them will suit the place they reach. Elm trees and oysters, which are sexual, produce millions of tiny young that drift on breezes or ocean currents until a few are lucky enough to land in a suitable place and begin a new life. Why do they do this? Because, said Williams, both elms and oysters have saturated their living space already. There are few clearings in an elm forest and few vacancies on an oyster bed. Each vacancy will attract many thousands of applicants in the form of new seeds or larvae. Therefore, it does not matter that your young are good enough to survive. What matters is whether they are the very best. Sex gives variety, so sex makes a few of your offspring exceptional and a few abysmal, whereas sex makes them all average.