Humanity is, of course, morally free to make and remake itself infinitely, but we do not do so. We stick to the same monotonously human pattern of organizing our affairs. If we were more adventurous, there would be societies without love, without ambition, without sexual desire, without marriage, without art. without grammar, without music, without st smiles—and with as many unimaginable novelties as are in that list. There would be societies in which women killed each other more often than men, in which old people were considered more beautiful than twenty-year olds, in which wealth did not purchase power over others, in which people did not discriminate in favor of their own friends and against strangers, in which parents did not love their own children.

It IS a mistake that biologists used to make, too. They believed that evolution proceeded by accumulating the changes that individuals gathered during their lives. The idea was most clearly formulated by Jean-Baptiste Lamarck, but Charles Darwin sometimes used it, too. The classic example is a blacksmith's son supposedly inheriting his father's acquired muscles at birth. We now know that Lamarckism cannot work because bodies are built from cakelike recipes, not architectural blueprints, and it is simply impossible to feed information back into the recipe by changing the cake.' But the first coherent challenge to Lamarckism was the work of a German follower of Darwin named August Weismann, who began to publish his ideas in the 18 80s.' Weismann noticed something peculiar about most sexual creatures: Their sex cells— ^ggs and sperm—remained segregated from the rest of the body from the moment of their birth. He wrote: "I believe that heredity depends upon the fact that a a small portion of the effective substance of the germ, the germ-plasm, remains unchanged during the development of the ovum into an organism, and that this part of the germ-plasm serves as a foundation from which germ-cells of the new organism are produced. There is, therefore, continuity of the germ-plasm from one generation to another.'

other words, you are descended not from your mother but "from her ovary Nothing that happened to her body or her mind in her life could affect your nature (though it could affect your nurture, of course—an extreme example being that her addiction to drugs or alcohol might leave you damaged in some nongenetic way at birth). You are born free of sin. Weismann was much ridiculed for this in his lifetime and little believed. But the discovery of the gene and of the DNA from which it is made and of the cipher in which DNA's message is written have absolutely confirmed his suspicion. The germ-plasm is kept separate from the body.

Every human being has two parents, four grandparents, eight great-grandparents, sixteen great-great-grandparents, and so on. A mere thirty generations back—in, roughly, A.D. 1066—you had more than a billion direct ancestors in the same generation (2 to the power of 30), Since there were fewer than a billion people alive at that time in the whole world, many of them were your ancestors two or three times over. If, like me, you are of British descent, the chances are that most all of the few million Britons alive in 1066, including King Harold, William the Conqueror, a random serving wench, and the meanest vassal (but excluding all well-behaved monks and nuns), are your direct ancestors. This makes you a distant cousin many times over of every other Briton alive today except the children of recent immigrants. All Britons are descended from the same set of people a mere thirty generations ago. No wonder there is a certain uniformity about the human (and every other sexual) species. Sex imposes it by its perpetual insistence on the sharing of genes.

If you go back further still, the different human races soon merge. Little more than three thousand generations back, all of our ancestors lived in Africa, a kw million simple hunter-gatherers, completely modern in physiology and psychology* As a result, the genetic differences between the average members of different races are actually tiny and are mostly confined to a few genes that affect skin color, physiognomy or physique. Yet the differences between any two individuals, of the same race or of different races, can still be large. According to one estimate, only 7 percent of the genetic differences between two individuals can be attributed to the fact that they are of different race; 85 percent of the genetic differences are attributable to mere individual variation, and the rest is tribal or national. In the words of one pair of scientists: "What this means is that the average genetic difference between one Peruvian farmer and his neighbor, or one Swiss villager and his neighbor, is twelve times greater than the difference between the 'average genotype of the Swiss population and the 'average genotype' of the Peruvian population."

One of the peculiar features of history is that time always erodes advantage. Every invention sooner or later leads to a counterinvention. Every success contains the seeds of its own overthrow. Every hegemony comes to an end. Evolutionary history is no different. Progress and success are always relative. When the land was unoccupied by animals, the first amphibian to emerge from the sea could get away with being slow, lumbering, and fishlike, for it had no enemies and no competitors. But if a fish were to take to the land today, it would be gobbled up by a passing fox as surely as a Mongol horde would be wiped out by machine guns. In history and in evolution, progress is always a futile, Sisyphean struggle to stay in the same relative place by getting ever better at things. Cars move through the congested streets of London no faster than horse-drawn carriages did a century ago. Computers have no effect productivity because people learn to complicate and repeat tasks that have been made easier.

This concept, that all progress is relative, has come to be known in biology by the name of the Red Queen, after a chess piece that Alice meets is in Through the Looking-Glass, who perpetually runs without getting very far because the landscape moves with her. It is an increasingly influential idea in evolutionary theory and one that will recur throughout the book. The faster you run, the more the world moves with you and the less you make progress. Life is a chess tournament in which it you win a game, you start the next game with the handicap of a missing pawn.

The relationship between a mother and her child is fairly straightforward: Both are seeking roughly the same goal—the welfare of themselves and each other. The relationship between a man and his wife's lover or between a woman and her rival for a promotion is also fairly Straightforward: Both want the worst for each other. One relationship is all about cooperation, the ot other all about conflict. But what is the relationship between a woman and her husband? It is cooperation in the sense that both want the best for the other. But why? In order to exploit each other. A man uses his wife to produce children for him. A woman uses her husband to make and help rear her children. Marriage teeters on the line between a cooperative venture and a form of n mutual exploitation—ask any divorce lawyer. S. Successful marriages so submerge the costs under mutual benefits that cooperation can predominate; unsuccessful ones do not.

A gazelle on the African savanna is trying not to be eaten by cheetahs, but it is also trying to outrun other gazelles when a cheetah attacks. What matters to the gazelle is being faster than other gazelles, not being faster than cheetahs. (There is an old story of a philosopher who runs when a bear charges him and his friend. "It's no good, you'll never outrun a bear," says the logical friend. "I don't have to." replies the philosopher. "I only have to outrun you.") In the same way, psychologists sometimes wonder why people are endowed with the ability to learn the part of Hamlet or understand calculus when neither skill was of much use to mankind in the primitive conditions where his intellect was shaped. Einstein would probably have been as hopeless as anybody in working out how to catch a woolly rhinoceros. Nicholas Humphrey, a Cambridge psychologist, was the first to see clearly the solution to this puzzle. We use our intellects not to solve practical problems but to outwit each other. Deceiving people, detecting deceit, understanding people's motives, manipulating people—these are what the intellect is used for. So what matters is not how clever and crafty you are but how much more clever and craftier you are than other people. The value of intellect is infinite. Selection within the species is always going to be more important than selection between the species.

Now this may seem a false dichotomy. After all, the best thing an individual animal can do for its species is to survive and breed. Often, however, the two imperatives will be in conflict. Suppose the individual is a tigress whose territory has recently been invaded by another tigress. Does she welcome the intruder and discuss how best they can cohabit the territory, sharing prey? No, she fights her to the death, which from the point of view of the species is unhelpful. Or suppose the individual is an eaglet of a rare species anxiously watched by conservationists in its nest. Eaglets often kill their younger brothers and sisters in the nest. Good for the individual, bad for the species.

Throughout the world of animals, individuals are fighting individuals, whether of the same species or of another. And indeed. the closest competitor a creature is ever likely to meet is a member of its own species. Natural selection is not going to pick genes that help gazelles survive as a species but hurt the chances of individuals—because such genes will be wiped out long before they can show their benefits. Species are not fighting species as nations battle other nations.

In recent years the geneticists have turned away from good mutations and begun to think about bad ones. Sex, they suggest, is a way of getting rid of bad mutations. This idea also has its origins in the 1960s, with Hermann Muller, one of the fathers of the Vicar of Bray theory. Muller, who spent much of his career at the University of Indiana, published his first scientific paper on genes in 1911, and a veritable flood of ideas and experiments followed in the succeeding decades. In 1964 he had one of his greatest insights; it has come to be known as "Mullet's ratchet." A simplified example of it goes like this: There are ten water fleas in a tank, only one of which is entirely free of mutations; the others all have one or several minor defects. On average only five of the water fleas in each generation manage to breed before they are eaten by a fish. The defect-free flea has a one-in-two chance of not breeding. So does the flea with the most defects, of course, but there is a difference: Once the defect-free flea is dead, the only way for it to be re-created is for another mutation to correct the mutation in a flea with a defect—a very unlikely possibility The one with two defects can be re-created easily by a single mutation in a water flea with one defect anywhere among its genes. In other words, the random loss of certain lines of descent will mean that the average number of defects gradually increases. Just as a ratchet turns easily one way but cannot turn back, so genetic defects inevitably accumulate. The only way to prevent the ratchet from turning is for the perfect flea to have sex and pass its defect-free genes to other fleas before it dies.

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.

Biologists have persistently overestimated the importance of physical causes of premature death rather than biological ones. In virtually any account of evolution, drought, frost, wind, or starvation looms large as the enemy of life. The great struggle, we are told, is to adapt to these conditions. Marvels of physical adaptation—the camel's hump, the polar bear's fur, the rotifer's boil-resistant tunare held to be among evolution's greatest achievements. The first ecological theories of sex were all directed at explaining this adaptability to the physical environment. But with the tangled bank, a different theme has begun to be heard, and in the Red Queen's march it is the dominant tune. The things that kill animals or prevent them from reproducing are only rarely physical factors. Far more often other creatures are involved—parasites, predators, and competitors. A water flea that is starving in a crowded pond is the victim not of food shortage but of competition. Predators and parasites probably cause most of the world's deaths, directly or indirectly When a tree falls in the forest, it has usually been weakened by a fungus. When a herring meets its end, it is usually in the mouth of a bigger fish or a in a net. What killed your ancestors two centuries or more ago? Smallpox, tuberculosis, influenza, pneumonia, plague, scarlet fever. diarrhea. Starvation or accidents may have weakened people, but infection killed them. A few of the wealthier ones died of old age or cancer or heart attacks, but not many.

Again and again in recent years evolutionary biologists have found themselves returning to the theme of parasites. As Is Richard Dawkins put it in a recent paper: "Eavesdrop [over] morning coffee at any major centre of evolutionary theory today, and you will find 'parasite' to be one of the commonest words in ti the language. Parasites are touted as the prime movers in the evolution of sex. promising a final solution to that problem of problems.'"

Parasites have a deadlier effect than predators for two reasons. One is that there are more of them. Human beings have no predators except great white sharks and one another, but they have lots of parasites. Even rabbits, which are eaten by stoats, weasels, foxes, buzzards, dogs, and people, are host to far more fleas, lice. ticks, mosquitoes, tapeworms, and uncounted varieties of protozoa. bacteria, fungi, and viruses. The myxomatosis virus has killed far more rabbits than have foxes. The second reason, which is the cause of the first, is that parasites are usually smaller than their hosts. while predators are usually larger. This means that the parasites live shorter lives and pass through more generations in a given time than their hosts. The bacteria in your gut pass through six times as many generations during your lifetime as people have passed through since they were apes.^^ As a consequence, they can multiply faster than their hosts and control or reduce the host population. The predator merely follows the abundance of its prey.

Parasites and their hosts are locked in a close evolutionary embrace. The more successful the parasite's attack (the more hosts it infects or the more resources it gets from each), the more the host's chances of survival will depend on whether it can invent a defense. The better the host defends, the more natural selection will promote the parasites that can overcome the defense. So the advantage will always be swinging from one to the other: The more dire the emergency for one, the better it will fight. This is truly the world of the Red Queen, where you never win, you only gain a temporary respite.

Many fungi are sexual, but they do not have males. They have tens of thousands of different sexes, all physically identical, all capable of mating on equal terms. but all incapable of mating with themselves. Even among animals there are many, such as the earthworm, that are hermaphrodites. To 3e sexual does not necessarily imply the need for sexes, let alone for just two sexes, let alone for two sexes as different as men and women. Indeed, at first sight, the most foolish system of all is two sexes because it means that fully 50 percent of the people you meet are incompatible as breeding partners. If we were hermaphrodites, everybody would be a potential partner. If we had ten thousand sexes, as does the average toadstool, 99 percent of those we meet would be potential partners. If we had three sexes, two-thirds would be available. It turns out that the Red Queen's solution to the problem of why people are sexual is only the beginning of a long story.

Another insect, a scale insect, has an even more bizarre genetic parasite. When its eggs are fertilized. sometimes more than one sperm penetrates the egg. If this happens, one of the sperm fuses with the egg's nucleus in the normal way; the spare sperm hang around and begin dividing as the egg divides. When the creature matures, the parasitic sperm cells eat out its gonads and replace them with themselves. So the insect produces sperm or eggs that are barely related to itself, an astonishing piece of genetic cuckoldry.

Trivers and Willard realized that the same general principle of sex allocation, which determines the gender of nematodes and fish, applies even to those creatures that cannot change sex but that take care of their young. They predicted that animals would be found to have some systematic control over the sex ratio of their own young. Think of it as a competition to have the most grandchildren. If males are polygamous, a successful son can give you far more grandchildren than a successful daughter, and an unsuccessful son will do far worse than an unsuccessful daughter because he will fail to win any mates at all. A son is a high-risk, high-reward reproductive option compared with a daughter. A mother in good condition gives her offspring a good start in life, increasing the chances of her sons' winning harems as they mature. A mother in poor condition is likely to produce a feeble son who will fail to mate at all, whereas her daughters can join harems and reproduce even when not in top condition. So you should have sons if you have reason to think they will do well and daughters if you have reason to think they will do poorly—relative to others in the population.

There are many well-established natural factors that bias the sex ratio of human offspring, proving that it is at least possible. The most famous is the returning-soldier effect. During and immediately after major wars, more sons are born than usual in the belligerent countries as if to replace the men that died. (This would make little sense; the men born after wars will mate with their contemporaries, not with those widowed by the war). Older fathers are more likely to have girls, but older mothers are more likely to have boys. Women with infectious hepatitis or schizophrenia have slightly more daughters than sons. So do women who smoke or drink. So did women who gave birth after the thick London smog of 1952. So do the wives of test pilots, abalone divers. rainfall for drinking water, there is a clear drop in the proportion of sons born 320 days after a heavy storm fills the dams and churns up the mud. Women with multiple sclerosis have more sons. as do women who consume small amounts of arsenic.

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.

There are five ways to find out. One is to study modern people directly and describe what they do as the human mating system The answer is usually monogamous marriage. A second way is to look at human history and divine from our past what sexual arrangements are typical of our species. But history teaches a dismal lesson: A common arrangement from our past was that rich and powerful men enslaved concubines in large harems. A third way is to look at people living in simple societies with Stone Age technologies and conjecture that they live much as our ancestors lived ten millennia ago. They tend to fall between the extremes: less polygamous than early civilizations, less monogamous than modern society. The fourth technique is to look at our closest relatives, the apes, and compare our behavior and anatomy with theirs. The answer that emerges is that men's testicles are not large enough for a system of promiscuity like the chimpanzee's, men's bodies are not big enough for a system of harem polygamy like the gorilla's (there is an iron link between harem polygamy in a species and a large size differential between male and female), and men are not as antisocial and adjusted to fidelity as the monogamous gibbon. We are somewhere in between. The fifth method is to compare humans with other animals that share our highly social habits: with colonial birds, monkeys, and dolphins. As we shall see, the lesson they teach is that we are designed for a system of monogamy plagued by adultery.

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.

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.

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.

There are three reasons to expect evolution to have produced different mentalities in men and women. The first is that men and women are mammals, and all mammals show sexual differences in behavior. As Charles Darwin put it, "No one disputes that the bull differs in disposition from the cow, the wild boar from the sow, the stallion from the mare."' The second is that men and women are apes, and in all apes there are great rewards for males that show aggression toward other males, for males that seek mating opportunities, and or females that pay close attention to their babies. The third is that men and women are human beings, and human beings are mammals with one highly unusual characteristic: a sexual division of labor Whereas a male and a female chimp seek the same sources of food, a male and a female human being, in virtually every preagricultural society, set about gathering food in different ways. Men look for sources that are mobile, distant, and unpredictable (usually meat), while women, burdened with children, look for sources that are static, close, and predictable (usually plants).

In other words, far from being an ape with fewer than usual sex differences, the human being may prove to be an ape with more than usual sex differences. Indeed, mankind may be the mammal with the greatest division of sexual labor and the greatest of mental differences between the sexes. Yet, though mankind may have added differences between the sexes. Yet, though mankind may have added division of labor to the list of causes of sexual dimorphism, he has subtracted the effect of male parental care.

Of the many mental features that are claimed to be different between the sexes, four stand out as repeatable, real, and persistent in all psychological tests. First, girls are better at verbal tasks. Second, boys are better at mathematical tasks. Third, boys are more aggressive. Fourth, boys are better at some visuo-spatial tasks and girls at others. Put crudely, men are better at reading a map and women are better judges of character and mood—on average. (And, interestingly, gay men are more like women than heterosexual men in some of these respects.)

The case of the visuo-spatial tasks is intriguing because it has been used to argue that men are naturally polygamous^ by analogy with the case of the mice quoted at the beginning of this chapter. Crudely put, polygamist mice need to know their way from one wife's house to another—and it is certainly true that in many polygamous animals, including our relatives, orangutans, males patrol an area that includes the territories of several wives. When people are asked to rotate a diagram of an object mentally to see if it is the same as another object, only about one in four women scores as highly as the average man. This difference grows during childhood. Mental rotation is the essence of map reading, but it seems a huge jump to argue that men are polygamous because they are better at map reading just because the same is true of mice.

Besides, there are spatial skills that women perform better than men. Irwin Silverman and Marion Eals at York University in Toronto reasoned that the male skill at mental-rotation tasks probably reflected not some parallel with polygamous male mice patrolling broad territories to visit many females but a much more particular fact about human history: that during the Pleistocene period, when early man was an African hunter-gatherer tor a million years or more, men were the hunters. So men needed superior spatial skills to throw weapons at moving targets, to make tools, to find their way home to camp after a long trek, and so on.

Much of this is conventional wisdom, but Silverman and Eals then asked themselves: What special spatial skills would women gatherers need that men would not? One thing they predicted was that women would need to notice things more—to spot roots, mushrooms, berries, plants—and would need to remember landmarks so as to know where to look. So Silverman and Eals did a series of experiments that required students to memorize a picture full of objects and then recall them later, or to sit in a room for three minutes and then recall what objects were where in the room. (The students were told they were merely being asked to wait in the room until a different experiment was ready.) On every measure of object memory and location memory, the women students did 60-70 percent better than the men. The old jokes about women noticing things and men losing things about the house and having to ask their wives are true. The difference appears around puberty, just as the social and verbal skills of women begin to exceed those of men.

We last met the steroid hormone testosterone in fish and birds where it was rendering them more vulnerable to parasites by exaggerating their sexual ornaments. In recent years more and more evidence has been found that testosterone affects not just ornaments and bodies but also brains. Testosterone is an ancient chemical. found in much the same form throughout the vertebrates. Its concentration determines aggressiveness so exactly that in birds with reversed sex roles, such as phalaropes and in female-dominated hyena clans, it is the females that have higher levels of testosterone in the blood. Testosterone masculinizes the body; without it, the body remains female, whatever its genes. It also masculinizes the brain.

Among birds it is usually only the male that sings. A zebra finch will not sing unless there is sufficient testosterone in its blood. With testosterone, the special song-producing part of its brain grows larger and the bird begins to sing. Even a female zebra finch will sing as long as she has been exposed to testosterone early in life and as an adult. In other words, testosterone primes the young zebra finch's brain to be responsive later in life to testosterone again and so develop the tendency to sing. Insofar as a zebra finch can be said to have a mind, the hormone is a mind-altering drug.

Much the same applies to human beings. Here the evidence comes from a series of natural and unnatural experiments. Nature has left some men and women with abnormal hormonal doses, and in the 1950s doctors changed the hormonal conditions of some wombs by injecting some pregnant women with certain hormones. Women with a condition known as Turner's syndrome (they are born without ovaries) have even less testosterone in their blood than do women who have ovaries. (Ovaries produce some testosterone, though not as much as testicles do.) They are exaggeratedly feminine in their behavior, with typically a special interest in Dabies, clothes, housekeeping, and romantic stories. Men with less than usual testosterone in their blood as adults—eunuchs, for example—are noted for their femininity of appearance and attitude. Men exposed to less than usual testosterone as embryos—for example, the sons of diabetic women who took female hormones during pregnancy—are shy, unassertive, and effeminate. Men with too much testosterone are pugnacious. Women whose mothers were injected with progesterone in the 1950s (to avert miscarriage) later described themselves as having been tomboys when young; progesterone is not unlike testosterone in its effects. Girls who were born with an unusual condition called either adrenogenital syndrome or congenital adrenal hyperplasia are equally tomboyish. This disorder causes the adrenal gland, near the kidney, to produce a hormone that acts like testosterone instead of Cortisol, its usual product."

Somewhat like in the zebra finches, there are two periods when testosterone levels rise in male children: in the womb, from about six weeks after conception, and at puberty. As Anne Moir and David Jessel put it in a recent book, Brain Sex, the first pulse of hormone exposes the photographic negative; the second develops it.'^ This is a crucial difference from the way the hormone affects the body. The body is masculinized by testosterone from the testides at puberty, whatever its womb experience. But not the mind. The mind is immune to testosterone unless it was exposed to a sufficient concentration (relative to female hormones) in the womb. It would be easy to engineer a society with no sex difference in attitude between men and women. Inject all pregnant women with th( right dose of hormones, and the result would be men and women with normal bodies but identical feminine brains. War, rape, boxing, car racing, pornography, and hamburgers and beer would soon be distant memories. A feminist paradise would have arrived.

At school, boys are fidgety, difficult, inattentive, and slow to learn, compared to girls. Nineteen out of every twenty hyperactive children are boys. Four times as many boys as girls are dyslexic and learning disabled. "Education is almost a conspiracy against the aptitudes and inclinations of a schoolboy," wrote psychologist Dianne McGuiness, a sentiment to which almost every man with a memory of school will raise a hearty cry of assent.

But another fact begins to emerge at school. Girls are simply better at linguistic forms of learning, boys at mathematical and some spatial skills. Boys are more abstract, girls more literal. Boys with an extra X chromosome (XXY instead of the normal XY) are much more verbal than other boys. Girls with Turner's syndrome (no ovaries) are even worse at spatial tasks than other girls but just as good at verbal ones. Girls who were exposed to male hormones in the womb are better at spatial tasks. Boys who were exposed to female hormones are worse at spatial tasks. These facts have been first disputed and then actively suppressed by the educational establishment, which continues to insist that there are no differences in learning ability between boys and girls. According to one researcher, such suppression has done both boys and girls far more harm than good.

It is clear, however, that the cause of homosexuality lies in some unusual balance of hormonal influence in the womb but not later on, a fact that further supports the idea that the mentality of sexual preference is affected by prenatal sex hormones. This is not incompatible with the growing evidence that homosexuality is genetically determined. The "gay gene" that I will discuss in the ext chapter is widely expected to turn out to be a series of genes that affect the sensitivity of certain tissues to testosterone. It is both nature and nurture.

It is no different from genes for height. Fed on identical diets, two genetically different men will not grow to the same height. Fed on different diets, two identical twins will grow to different heights. Nature is the length of the rectangle, nurture the width. There can be no rectangle without both. The genes for height are really only genes for responding to diet by growing.

Hard porn, which depicts actual acts of sex, is almost invariably about the gratification of male lust by willing, easily aroused, varied, multiple, and physically attractive women (or men, in the case of gay porn). It is virtually devoid of context, plot, flirtation, courtship, and even much foreplay. There are no encumbering relationships, and the coupling duo are usually depicted as strangers. When two scientists showed heterosexual students pornographic films and measured their arousal by them, they found a consistent pattern of the kind common sense would suggest. First, men were more aroused than women. Second, men were aroused more by depictions of group sex than by films of a heterosexual couple, whereas for women it was the other way around. Third, women and men were both aroused by lesbian scenes, but neither was aroused by male homosexual sex. (Remember, all these students were heterosexual.) When watching pornography, men and women are both interested in the women actors. But porn is designed for, marketed to, and sought out by men, not women.

The romance novel, by contrast, is aimed entirely at a female market. It, too, depicts a fictional world that has changed remarkably little except in adapting to female career ambitions and to a less inhibited attitude toward the description of sex. Authors adhere strictly to a formula provided by the publishers. Sexual acts play a small part in these novels; the bulk of each book is about love, commitment, domesticity, nurturing, and the formation of relationships. There is little promiscuity or sexual variety, and what sex there is, is described mainly through the heroine's emotional reaction to what is done to her—particularly the tactile things— and not to any detailed description of the man's body. His character is often discussed in detail but not his body.

...the concept of "neoteny"—the retention of juvenile features into adult life. It is a commonplace of human evolution that the transition from Australopithecus to Homo and from Homo habilis to Homo erectus and thence to Homo sapiens all involved prolonging and slowing the development of the body so that it still looked like a baby when it was already mature. The relatively large brain case and small jaw, the slender limbs, the hairless skin, the unrotated big toe, the thin bones, even the external female genitalia—we look like baby apes.

The skull of a baby chimpanzee looks much more like the skull of an adult human being than either the skull of an adult chimpanzee or the skull of a baby human being. Turning an apeman into a man was a simple matter of changing the genes that affect the rate of development of adult characters, so that by the time we stop growing and start breeding, we still look rather like a baby. "Man is born and remains more immature and for a longer period than any other animal," wrote Ashley Montagu in 1961.

The evidence for neoteny is extensive. Human teeth erupt through the jaw in a set order: the first molar at the age of six, compared with three for a chimp. This pattern is a good indication of all sorts of other things because the teeth must come at just the right moment relative to the growth of the jaw. Holly Smith, an anthropologist at the University of Michigan, found in twenty-one species of primate a close correlation between the age at which the first molar erupted and body weight, length of gestation, age at weaning, birth interval, sexual maturity, life span, and especially brain size. Because she knew the brain size of fossil hominids, she was able to predict that Lucy would have erupted her first molar at three and lived to forty, much like chimpanzees, whereas the average Homo erectus would have erupted his at nearly five and lived to fifty-two.

Likewise, the manner in which sexual selection capriciously seizes upon preexisting perceptual biases fits with the fact that apes are by nature naturally "curious, playful, easily bored, and appreciative of simulation." Miller suggests that to keep a husbanc around long enough to help in raising children, women would have needed to be as varied and creative in their behavior as possible, which he calls the Scheherazade effect after the Arabian storyteller who entranced the Sultan with 1,001 tales so that he did not abandon her (and execute her) for another courtesan. The same would have applied to males who wanted to attract females, which Miller calls the Dionysus effect after the Greek god of dance, music, intoxication, and seduction. He might also have called it the Mick Jagger effect; he admitted to me one day that he could not understand what made strutting, middle-aged rock stars so attractive to women. In this respect Don Symons noted that tribal chiefs are Doth gifted orators and highly polygamous men.

This book is crammed with original ideas—very few of them my own. Science writers become accustomed to the feeling that they are intellectual plagiarists, raiding the minds of those who are too busy to tell the world about their discoveries. There are scores of people who could have written each chapter of my book better than I. My consolation is that few could have written all the chapters. My role has been to connect the patches of others' research together into a quilt.

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.

If we were hairless chimpanzees, our society would still Dok fairly familiar in some ways. We would live in families, be very social, hierarchical, group-territorial, and aggressive toward other groups than those we belong to. In other words, we would be family-based, urban, class-conscious, nationalist, and belligerent, which we are. Adult males would spend more time trying to climb the political hierarchy than with their families. But when we turn to sex, things would begin to look very different. For a start, men would take no part at all in rearing the young, not even paying child support; there would be no marriage bonds at all. Most women would mate with most men, though the top male (the president, let us call him) would make sure he had droit du seigneur over ! the most fertile women. Sex would be an intermittent affair, indulged in to spectacular excess during the woman's estrus but totally forgotten by her for years at a time when pregnant or rearing a young child. This estrus would be announced to everybody in sight by her pink and swollen rear end, which would prove irresistibly fascinating to every male who saw it. They would try to monopolize such females for weeks at a time, forcing them to go away on a "consortship" with them; they would not always succeed and would quickly lose interest when the swelling went down. Jared Diamond of the University of California at Los Angeles has speculated on how disruptive this would be to society by imagining the effect on the average office of a woman turning up for work one day irresistibly pink.

Mankind is a self-domesticated animal; a mammal; an ape; a social ape; an ape in which the male takes the initiative in courtship and females usually leave the society of their birth; an ape in which men are predators, women herbivorous foragers; an ape in which males are relatively hierarchical, females relatively egalitarian; an ape in which males contribute unusually large amounts of investment in the upbringing of their offspring by provisioning their mates and their children with food, protection, and company; an ape in which monogamous pair bonds are the rule but many males have affairs and occasional males achieve polygamy; an ape in which females mated to low-ranking males often cuckold their husbands in order to gain access to the genes of higher-ranking males; an ape that has been subject to unusually intense mutual sexual selection so that many of the features of the female body (lips, breasts, waists) and the mind of both sexes (songs, competitive ambition. status seeking) are designed for use in competition for mates; an ape that has developed an extraordinary range of new instincts to learn by association, to communicate by speech, and to pass on traditions. But still an ape.

Why does the waist-to-hip ratio matter? Singh observes that a "gynoid" fat distribution—more fat on the hips, less on the torso—is necessary for the hormonal changes associated with female fertility. An "android" fat distribution-—fat on the belly, thin hips—is associated with the symptoms of male disabilities such as heart disease, even in women. But which is cause and which effect? It seems to me more likely that both the shape and the hormonal effects of it are sexually selected by generations of males rather than males preferring the shape because it is the only way the hormones can be made to work. The relatively brief period during which women have hourglass bodies—from fifteen to thirty-five, say—is a sexually selected phenomenon. It owes more to competition to attract men than to any other biological need. Men have been unconsciously acting as selective breeders of women.

Low provides one possible reason for the male preference tor a low ratio—choosing broad-hipped women more able to give birth. Most apes give birth to babies whose brain is half-grown; human babies' brains are one-third grown at birth, and they spend far less time in the womb than is normal for a mammal, given the longevity of man. The reason is obvious: Were the hole in the pelvis through which we are born (the birth canal) commensurately larger, our mothers would be unable to walk at all. The width of human hips reached a certain point and could go no further; as brains continued to grow bigger, earlier birth was the only option left to the species. Imagine the evolutionary pressure of this process on female hip size. It was always wise for a man to choose the biggest-hipped woman he could find, generation after generation, for millions of years. At a certain point hips could get no bigger but men still had the preference, so women with slender waists who appeared to have larger hips by contrast were preferred instead.

Then for more than a million years people lived in a way that couldn't have changed much. They inhabited grasslands and woodland savannas, first in Africa, later in Eurasia, and eventually in Australasia and the Americas. They hunted animals for food, gathered fruits and seeds, and were highly social within each tribe but hostile toward members of other tribes. Don Symons refers to this combination of time and place as the "environment of evolutionary adaptedness," or EEA, and he believes it is central to human psychology. People cannot be adapted to the present or the future; they can only be adapted to the past. But he readily admits that it is hard to be precise about exactly what lives people lived in the EEA. They probably lived in small bands; they were perhaps nomadic; they ate both meat and vegetable matter; they presumably shared the features that are universal among modern humans of all cultures: a pair bond as an institution in which to rear children, romantic love, jealousy and sexually induced male violence, a female preference for men of high status, a male preference for young females, warfare between bands, and so on. There was almost certainly a sexual division of labor between hunting men and gathering women, something unique to people and a few birds of prey. To this day, among the Ache people of Paraguay, men specialize in acquiring those foods that a woman encumbered with a baby could not manage to—meat and honey. for example."

Kim Hill, at the University of New Mexico, argues that there was no consistent EEA, but he nonetheless agrees that there were universal features of human life that are not present today but that have hangover effects. Everybody knew or had heard of nearly all the people they were likely to meet in their lives: There were no strangers, a fact that had enormous importance for the history of trade and crime prevention, among other things. The lack of anonymity meant that charlatans and tricksters could rarely get away with their deceptions for long.

Another group of biologists at Michigan rejects these EEA arguments altogether with two arguments. First, the most critical feature of the EEA is still with us. It is other people. Our brains grew so big not to make tools but to psychologize one another. The lesson of socioecology is that our mating system is determined not by ecology but by other people—by members of the same gender and by members of the other gender. It is the need to outwit and dupe and help and teach one another that drove us to be ever more intelligent.

Second, we were designed above all else to be adaptable. We were designed to have all sorts of alternative strategies to achieve our ends. Even today, existing hunter-gatherer societies show enormous ecological and social variation, and they are probably an unrepresentative sample because they mostly occupy deserts and forests, which were not mankind's primary habitat. Even in the time of Homo erectus, let alone more modern people, there may have been specialized fishing, shore-dwelling, hunting, or plant-gathering cultures. Some of these may well have afforded opportunities for wealth accumulation and polygamy. In recent memory there was a preagricultural culture among the salmon-fishing Indians of the Pacific Northwest of America that was highly polygamous. If the local hunter-gathering economy favored it, men were capable of I being polygamous and women were capable of joining harems over the protests of the preceding co-wives. If not, then men were capable of being good fathers and women jealous monopolizers. In other words, mankind has many potential mating systems, one for each circumstance.

This is supported by the fact that larger, more intelligent and more social animals are generally more flexible in their mating systems than smaller, dumber, or more solitary ones. Chimps go from small feeding bands to big groups depending on the nature of the food supply. Turkeys do the same. Coyotes hunt in packs when their food is deer but hunt alone when their food is mice. These food-induced social patterns themselves induce slightly different mating patterns.

Finding the right balance between cooperation and competition has been the goal and bane of Western politics for centuries. Adam Smith recognized that the economic needs of the individual are better met by unleashing the ambitions of all individuals than by planning to meet those needs in advance. But even Adam Smith could not claim that free markets produce Utopia. Even the most libertarian politician today believes in the need to regulate, oversee, and tax the efforts of ambitious individuals so as to ensure that they do not satisfy their ambitions entirely at the expense of others. In the words of Egbert Leigh, a biologist at the Smithsonian Tropical Research Institute, "Human intelligence has yet to design a society where free competition among the members works for the good of the whole." The society of genes faces exactly the same problem. Each gene is descended from a gene that unwittingly jostled to get into the next generation by whatever means was in its power. Cooperation between them is marked, but so is competition. And it is that competition that led to the invention of gender.

The immune system consists of white blood cells that come in about 10 million different types. Each type has a protein lock on it called an "antibody," which corresponds to a key carried by a bacterium called an "antigen." If a key enters that lock, the white cell starts multiplying ferociously in order to produce an army of white cells to gobble up the key-carrying invader, be it a flu virus, a tuberculosis bacterium, or even the cells of a transplanted heart. But the body has a problem. It cannot keep armies of each antibody-lock ready to immobilize all types of keys because there is simply no room for millions of different types, each represented by millions of individual cells. So it keeps only a few copies of each white cell. As soon as one type of white cell meets the antigen that fits its locks, it begins multiplying. Hence the delay between the onset of flu and the immune response that cures it.

Each lock is generated by a sort of random assembly device that tries to maintain as broad a library of kinds of lock as it can, even if some of the keys that fit them have not yet been found in parasites. This is because the parasites are continually changing their keys to try to find ones that fit the host's changing locks. The immune system is therefore prepared. But this randomness means that the host is bound to produce white cells that are designed to attack its own cells among the many types it invents. To get around this, the host's own cells are equipped with a password, which is known as a major histocompatibility antigen. This stops the attack.

To win, then, the parasite must do one of the following: infect somebody else by the time the immune response hits (as flu does), conceal itself inside host cells (as the AIDS virus does), change its own keys frequently (as malaria does), or try to imitate whatever password the host's own cells carry that enable them to escape attention. Bilharzia parasites, for example, grab password molecules from host cells and stick them all over their bodies to camouflage themselves from passing white cells. Trypanosomes, which cause sleeping sickness, keep changing their keys by switching on one gene after another. The AIDS virus is craftiest of all. According to one theory it seems to keep mutating so that each generation has different keys. s. Time after time the host has locks that fit the keys and the virus gets suppressed. But eventually, after perhaps ten years, the virus's random mutation hits upon a key that the host does not have a lock for. At that point the virus has won. It has found the gap in the repertoire of the immune system's locks and runs riot. In essence, according to this theory the AIDS virus evolves until it finds a chink in the body's immune armor.

Michael Ghiselin developed this idea further in 1974 and made some telling analogies with economic trends. As Ghiselin put it, "In a saturated economy, it pays to diversify." Ghiselin suggested that most creatures compete with their brothers and sisters, so if everybody is a little different from their brothers and sisters, then more can survive. The fact that your parents thrived doing one thing means that it will probably pay to do something else because the local habitat might well be full already with your parents' friends or relatives doing their thing.

Graham Bell has called this the "tangled bank" theory, after the famous last paragraph of Charles Darwin's Origin of Species: "It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other and dependent upon each other in so complex a manner, have all been produced by laws acting around us."

If sex had no cost, hummingbirds would not exist. Hummingbirds eat nectar, which is produced by flowers to lure pollinating insects and birds. Nectar is a pure gift by the plant of its hard-won sugar to the hummingbird, a gift given only because the hummingbird will then carry pollen to another plant. To have sex with another plant, the first plant must bribe the pollen carrier with nectar. Nectar is therefore a pure, unadulterated cost incurred by the plant in its quest for sex. If sex had no cost. there would be no hummingbirds.

Inside my skull is a brain that was designed to exploit the conditions of an African savanna between 3 million and 100,000 years ago. When my ancestors moved into Europe (I am a white European by descent) about 100,000 years ago, they quickly evolved a set of physiological features to suit the sunless climate of northern latitudes: pale skin to prevent rickets, male beards, and a circulation relatively resistant to frostbite. But little else changed: Skull size, body proportions, and teeth are all much the same in me as they were in my ancestors 100,000 years ago and are much the same as they are in a San tribesman from southern Africa. And there is little reason to believe that the gray matter inside the skull changed much, either. For a start, 100,000 years is only three thousand generations, a mere eye blink in evolution, equivalent to a day and a half in the life of bacteria. Moreover, until very recently the life of a European was essentially the same as that of an African. Both hunted meat and gathered plants. Both lived in social groups. Both had children dependent on their parents until their late teens. Both used stone, bone, wood, and fiber to make tools. Both passed wisdom down with complex language. Such evolutionary novelties as agriculture, metal. and writing arrived less than three hundred generations ago, far too recently to have left much imprint on my mind.

There is, therefore, such a thing as a universal human nature, common to all peoples. If there were descendants of Homo erectus still living in China, as there were a million years ago, and those people were as intelligent as we are, then truly they could be said to have different but still human natures.'* They might perhaps lave no lasting pair bonds of the kind we call marriage, no concept of romantic love, and no involvement of fathers in parental care. We could have some very interesting discussions with them about such matters. But there are no such people. We are all one close family, one small race of the modern Homo sapiens people who lived in Africa until 100,000 years ago, and we all share the nature of that beast.

Those strains that reproduce persist; those that do not reproduce die out. The ability to reproduce is what makes living things different from rocks. Besides, there is nothing inconsistent with free will or even chastity in this view of life. Human beings, I believe, thrive according to their ability to take initiatives and exercise individual talent. But free will was not created for fun; there was a reason that evolution handed our ancestors the ability to take initiatives, and the reason was that free will and initiative are means to satisfy ambition, to compete with fellow human beings, to deal with life's emergencies, and so eventually to be in a better position to reproduce and rear children than human beings who do not reproduce. Therefore, free will itself is any good only to the extent that it contributes to eventual reproduction.

Look at it another way: If a student is brilliant but terrible in examinations—if, say, she simply collapses with nervousness at the very thought of an exam—then her k brilliance will count for nothing m a course that is tested by a single examination at the end of the term. Likewise, if an animal is brilliant at survival, has an efficient metabolism, resists all diseases, learns faster than its competitors, and lives to a ripe old age, but is infertile, then its superior genes are simply not available to its descendants. Everything can be inherited except sterility. None of your direct ancestors died childless.