Two of the memorably worst lines of English poetry, composed in 1799 by one John Hookham Frere:

The feather’d race with pinions skim the air—
Not so the mackerel, and still less the bear.

Hookham Frere’s insensitivity to bathos is impressive; but however inferior his verse it has a moral. The lines come from his unduly neglected poem The Progress of Man; Poetry of the Anti-Jacobin. Birds, bears, and fish carry a political message: Things are as they are and it is folly to change them. The French Revolution disturbed the God-given order: to proclaim the rights of man was as absurd as to suggest that mackerel—or even bears—might fly.

That logic is still with us. Nowadays, though, the status is quo because of evolution rather than the Almighty. Darwinism’s ability to explain the human condition was seized upon as soon as The Origin of Species appeared. From Plato to Galton, from Bernard Shaw to Charles Murray, biology has a dismal history of being used to absolve murderers, rapists, thieves, martyrs, saints, capitalists, Communists, and social democrats from blame for their actions.

Jared Diamond’s book sets out to evolutionize sex. It pins its heart (a metaphorical one in this case) to its sleeve in the very first paragraph. Sex, Diamond says, “…is the source of our most intense pleasures. Often it’s also the cause of misery, much of which arises from built-in conflicts between the evolved roles of women and men.”

So much, then, for the Arts Faculty; so simple the logic behind literature, painting, and the stage. Sex is, in addition, claims Diamond, responsible for the origin of hairlessness and tool making. It is all, it seems, a matter of evolved roles.

When it comes to the mating game, humans are unique. What causes women to be the only female mammals to conceal the time at which they are most fertile? Why, when it comes to penis size, does man stand alone? What is behind the human menopause, shared only with the pilot whale? Why, for that matter, is voyeurism not even commoner than the Internet allows? Other mammals do it in the road; and human sex is most peculiar in not being a social activity. We may share 98.8 percent of our genes with chimps and almost as many with gorillas; but when it comes to matters reproductive we are very different from either. The sex lives of most readers of The New York Review of Books are more similar (I assume) to those of albatrosses than of chimpanzees. Humans and sea birds both live in large colonies but show fidelity, albeit a grudging one, to a single mate. No other primate is remotely like that.

Diamond’s case is that our sex lives are better understood in terms of genetics than of grand opera. He is among the best of a talented bunch of writers on evolutionary biology. This book is (like his earlier work on the same theme, The Rise and Fall of the Third Chimpanzee) engaging and interesting to read. It is much more than—yet another—infantile attempt to use a string of unconnected anecdotes to explore the human spirit through the eyes of nature. Why Is Sex Fun? contains many strange and memorable tales of our sex lives and those of our relatives. It is guaranteed to keep a potential partner amused and amazed at the vagaries of what people—and animals—get up to when it comes to passing on their genes.

However, even Diamond falls into the trap that faces all who use Darwin to rationalize the human condition. No one denies that human behavior is constrained by our evolutionary past, in the sense that a pig’s ability to fly is limited by its ancestors’ lack of wings. It is fatally easy, though, to rework biology to fit a cultural idea; and by explaining everything to explain nothing.

The Origin was published only because Darwin received a letter containing the same notion from Alfred Russel Wallace. Their theory was presented to the Linnaean Society of London in 1858. It had rather little impact. The president (a dentist interested in reptiles) claimed that the year had not “been marked by any of those striking discoveries which at once revolutionize, so to speak, the department of science on which they bear; it is only at remote intervals that we can reasonably expect any sudden and brilliant innovation which shall produce a marked and permanent impression on the character of any brand of knowledge.”

That lack of judgment is reflected by the president’s own book, Kalygonomia, or the Laws of Female Beauty, an early contribution to the scientific study of sex. In those pre-Darwinian days, its main conclusion was to list “Defects in the intellectual system of Woman (4); Defects in the Mechanical system of Women (17) and Defects in the Vital system of Women (9).” The reptilian dentist had, alas, failed to notice the alibi which the society’s two speakers were to provide all possible theories of human sexuality. His work is forgotten.


Evolution is to allegory as statues are to birdshit. It is a convenient platform upon which to deposit badly digested ideas. Darwin saw where the importance of his theory really lay (“species are not—it is like confessing a murder—immutable”). He was much opposed to its naive use in human affairs. Darwinism has, though, been debased since it began by those who use it to support one creed or another. The problem goes far beyond sex. F.R. Leavis derided “the culture of the Sunday papers.” There is a modern counterpart. It is the culture of Scientific American: using a nodding acquaintance with evolution to promote an ethical agenda.

Diamond himself is far too good a biologist to fall into the trap of making science serve only as fuel for metaphor, although his book will no doubt be hijacked by those who do. Before accepting his arguments about the origins of human sex, though, it is worth remembering how dangerously accommodating evolutionary theory can be.

Alfred Russel Wallace became a socialist. He saw what natural selection’s message really was:

All shall contribute their share either of physical or mental labour, and…every one shall obtain the full and equal reward for their work. The future progress of the race will be rendered certain by the fuller development of its higher nature acted on by a special form of selection which will then come into play.

Herbert Spencer, Darwin’s amanuensis and inventor of that unfortunate phrase “the survival of the fittest,” came to a different conclusion. His notion of Social Darwinism was used to justify the excesses of capitalism. One of its proponents saw millionaires as “naturally selected in the crucible of competition.” The steel magnate Andrew Carnegie agreed. “Before Spencer,” he said, “all for me had been darkness, after him, became light—and right.”

According to Jared Diamond, Darwinism as light and right illuminates the secret crannies of human sexuality. He gives, for example, a giraffe’s-eye view of the child support laws. Males should, in any sensible mammal society, be off spreading their genes rather than squandering further efforts on an investment already made. Most are. What seems to us natural and decent, the nuclear family, would appear perverse to our relatives (let alone to the giraffe, whose long neck, according to the latest theory, is a product of males competing to impress females rather than stretching for the highest leaf). The family is as much a product of evolution as is a mandrill’s bottom: each is no more than a mechanism for ensuring the efficient transmittal of DNA. Men stay home only because human children—and the genes they contain—need two parents to have a good chance of surviving.

Evolution also explains why mothers are kinder to their babies than are fathers. It is only because they have already put so much into bearing them in the first place. Stepmothers are wicked for Mendelian reasons: they abuse their partner’s children because they are the vehicles of another woman’s genes.

All this is familiar enough. Evolution explains many strange patterns of animal behavior. Why should it not do the same for us? For a male spider who is going to meet—and have a chance to mate with—one female, it makes perfect sense to offer himself as a nuptial meal. It is all a matter of investment: feed your mate rather than start a hopeless search for another. When it comes to the human condition, as Diamond points out, a woman (however consumed by desire she might be) could not chew her way through enough of a man’s body at one sitting to make much difference to her baby’s survival. Perhaps, though, if men had only one chance for sex, post-copulatory cannibalism could catch on. Evolution has done stranger things.

As a professional biologist with a vast range of expertise, Jared Diamond is able to pull some startling skeletons out of the human sexual cupboard. Why, for example, is the milk of human kindness a product of only half the population? After all, men have nipples and the potential to use them. Males given certain chemicals to treat cancer lactate quite readily. Even a dose of heavy drinking can do the same thing as the liver loses its ability to suppress every man’s guilty secret, his circulating female hormones.

Women who have adopted a child (and who are not primed by pregnancy) can sometimes produce milk when suckled. Teenage boys, in a natural desire to see what might be done with their bodies, now and then take to stimulating their own nipples and (no doubt to their surprise) may succeed in doing the same. However, only the Dyak fruit bat goes to the logical conclusion. Males not only make milk but suckle their young. Given that men invest lots of care in their children (including earning enough to take them to McDonald’s), it is strange that they do not go the whole hog—or fruit bat—and produce food directly.


There is, as usual in biology, both an immediate and an ultimate explanation. The immediate reason why men have no milk has to do with DNA: males have the Y chromosome that is the switch that—just like those outside Paddington Station that with one tiny move command a train to go to Oxford or to Penzance—directs the embryo, within its first few hours, to develop as a male rather than a female. The Y carries one simple gene. The essence of maleness lies in its 240 DNA bases. All they do is increase the rate of growth of the early embryo, and by so doing to persuade it to make testes. From those simple orbs great consequences flow, among them pert but useless nipples.

Men—male mammals in general—have the potential to make milk but (pace the fruit bat) they do not. That is where evolution comes in. First, males in most species of mammal are crass enough to leave at fertilization rather than hang around for birth. Their energies are better invested in more sperm and more mates than in feeding the results of an earlier fling. There is simply no market for their milk.

Even for that noble minority of species (humans included) in which males stay to support their mates, a division of labor is called for. Male nipples have the same history as does the nuclear family. In mammals (unlike birds, whose eggs are laid soon after fertilization and can be incubated with equal success by either parent) the female is already committed—through a long and expensive pregnancy—to invest in her child. However feeble the child might be, it will rarely pay her to abandon it in the hope of doing better with another mate. Instead, she must throw good eggs after bad by feeding her offspring. Natural selection has turned her into a milk-producing machine. The male’s job is to protect the child while keeping a weather eye open for something better. He does so by hunting, guarding the marital home, or (in marmosets) carrying one identical twin while his mate suckles the other.

According to Diamond, men are—more than any other male mammal—ecologically primed to be breast-feeders. They are reasonably monogamous (and hence confident that any child is indeed theirs) and children take years to wean. Perhaps, he suggests, in this technical age, the next fad will be male breast-feeding, brought on with a simple hormone injection. Think how much better a child would bond with its father; how excellent breast milk is as a food; how it might improve a wife’s career. We already reject rape and genocide, which, from a naive evolutionary perspective, favor our genes. The next choice may be for the New Man to suckle his child. No doubt he will do it in public.

Diamond moves on to another bafflingly redundant part of man’s anatomy. The penis is what raises us above our fellow primates. It is, comparatively speaking, huge. The gorilla has a guilty secret: in spite of its pectorals it has a one-and-a-quarter-inch member. The chimpanzee, a copulator of gigantic appetite (a male mates hundreds of times with dozens of females each year), does little better.

Why the uniquely large human penis? Diamond suggests that it results from “runaway selection”: an idea first put forward to explain structures such as the peacock’s tail. For some unknown (and perhaps arbitrary) reason, in the ur-peacock long ago, females preferred males with slightly longer tails. Such males hence had more offspring: sons with showier tails, and daughters of refined aesthetic taste. The process fed on itself until, in time, a bizarre and apparently useless appendage evolved. Its size was limited only by the disadvantage of having to cart around a vast accessory whose only job is to impress a potential mate.

Perhaps that is what caused the human penis to expand: it simply ran away with itself. A penis does not, it must be said, cost as much in metabolic terms as a peacock’s tail; but, as Jared Diamond points out, think how much more useful it would be if all that too solid flesh were to be recycled into brain or muscle. That unique organ might all be a matter of female choice (although even Diamond balks at discussing Richard Dawkins’s idea that the rigidity of an erection allows her to test the physical fitness and genetic quality of a partner). Oddly enough, the same argument has been used to explain the peacock’s complete lack of a penis. Males were shorn of their appendage, it is said, because the females chose those with small ones.

There is, it seems, a natural experiment which tests Diamond’s idea. In New Guinea, men of the Ketengban tribe enhance what nature has provided by wearing a brightly decorated pointed gourd—a phallocarp—upon it. Without this additional foot or so of reassurance, they say, they feel naked. Perhaps, suggests Diamond, this is what the penis would evolve into were it not constrained by some opposing force of natural selection. It is, above all, a signal of virility.

Ingenious though this is, it illustrates the dangers of applying evolution to human behavior. Sexual conduct is so polymorphously perverse that it provides an example to fit almost any theory. In most places, of course, the penis is not a signal of social or genetic excellence at all. Instead, it is hidden away until the last possible moment. Even Gordon Gekko did not wear a phallocarp. The Duke of Edinburgh, on a visit to New Guinea long ago, was given one; but he has consistently refused to reveal whether he has ever put it on. In spite of the rumors of strategically placed rubber hoses that attached themselves to Mick Jagger, most Western males follow in the ducal path; they are proud of their appendage, but only in private. There is, as far as I know, no difference in penis size between societies that flaunt the male organ and those that conceal it; but since both exist it is fatally easy to appeal to the one that fits the theory.

The dangerous flexibility of the penis argument extends to other aspects of sexual life. One of its odder attributes is that women hide the time at which they are most fertile. That gives Diamond’s book its title. Most sex is perforce for fun as, most of the time, females have not produced an egg to be fertilized. Unique among mammals, women are the shy sex. Instead of virtuously flaunting their availability by, say, baring their buttocks on fecund days (as any self-respecting primate would), they conceal it—and so effectively that many are not instinctively conscious of when they themselves have ovulated. For a supposedly self-aware creature, this is embarrassing. After all, cows know; why not us?

As so often in human affairs, there is an excess of speculation over fact. Two perfectly opposed hypotheses exist. Each can be supported by choosing the right evidence. One has it that a female who advertises her fertility faces the danger of her husband wandering off to find another mate. He knows when she cannot conceive, and for much of the time need not worry if she falls for a passing male. On most days of the month it pays him to search for sex elsewhere instead of doing his family duties. Concealed ovulation prevents him from knowing what days they are and forces him to stay at home.

The other idea could not be more different. In many creatures, infanticide is common; in gorillas, for instance, it causes a third of infant deaths. A male stumbles upon a litter that is not his and kills it to make the female available as a receptacle for his own genes. Humans do not do this because of concealed ovulation. In a promiscuous society in which a woman who mates with many men hides her most fertile time, none of her partners is certain whether her child is his own. As a result, all refrain from murder.

Which is true, concealed oestrus as a device to promote monogamy (father stays home) or to overturn it (the killing of babies means that monogamy is not a genetically safe way of life)? Primates are little help: concealed ovulation has appeared and disappeared several times in different lineages. Diamond settles for a messy compromise, in which sex as fun started in order to avoid infanticide, and ended as monogamy insurance. It is hard to know what could disprove such a catholic theory.

Jared Diamond is not the only scientist to fall too easily into the capacious arms of natural selection. The whole of biology, from the structure of DNA to the origin of species, is riven by argument about just how important it is. Only too often, a failure to find evolutionary adaptation at work in one place is explained away by saying that it must be toiling away somewhere else. Nowhere is that more true than when science meets human affairs.

Take the role of men in child care. That once seemed obvious; they hunt, bringing back food to share with those who carry their genes. Unfortunately, they don’t: studies of most hunting societies show that men would do better as providers by toiling in fields with the women. When they do kill, the food often goes to male friends rather than to wives and offspring. Kin selection—which explains the lives of bees very nicely—does not work.

However, stir in another evolutionary nostrum, reciprocal altruism (I feed you today, you feed me tomorrow), and the recipe regains its flavor. If the anthropological soufflé still fails to rise, reach for the bottle labeled “differential reproductive success”: the hunter rarely scores the big one, but when he does, he has meat to give away and (naturally) the females lay down for him in droves. So finely tuned is the society of the Aché of West Africa, apparently, that even the relative paucity of hunters (the Donald Trumps of the jungle world) is explained. Too many Trumps spoil the broth: females get too choosy and it is no longer worth making the effort of chasing some large and dangerous animal in the hope of sexual success.

Just as in a soufflé, the numbers and proportions of ingredients can be varied according to taste or to circumstance; but mix them enthusiastically enough and, with one bound, society is explained. It’s the kind of thing that gives evolution a bad name.

In biology, sometimes, things just happen. They do not demand the hidden hand of adaptation. The females of a certain moth and of the African elephant produce exactly the same complex chemical, a sexual signal known as a pheromone. This attracts males of either species (which must be riskier for the moth than for the other partner in the relationship). One could, no doubt, think up an evolutionary hypothesis: the moth lays eggs in elephant dung (it doesn’t), the elephant has been infected by moth DNA (unlikely). None is called for: evolution is so protean in its doings that it will sometimes do the same thing twice, just by chance. No special explanation is required.

That, surely, is true for much of our sexual behavior. It is incidental to the healing power of lust: there is no need to justify each of its infinite varieties with an adaptive story. Sex is fun, certainly, but do we always need Darwin to tell us why? The most remarkable thing about human evolution is how little there has been. In the past hundred thousand—in the past hundred—years, our lives have been transformed but our bodies have not. Humans are remarkable in the number of things they do and in how little they are specialized to do them. Our evolution is unique in that it is mainly in the mind. The human species has not undergone adaptive radiation. Instead, technology—and society—has done the adaptation for us.

Even in the past century there have been great changes in sexual strategy. In Britain they shifted from the secret polygamy of the Victorian upper classes with their army of prostitutes to the dull fidelity of the postwar family, through what many unreliably recall as the riotous promiscuity of the Sixties to (at least as far as my own students are willing to admit) something close to serial monogamy today. These changes could be argued to be mere froth on the surface of an evolved sexual strategy—but in 0.001 percent of our history: some froth!

Jared Diamond’s book is an eloquent statement of the power of evolution, written by a virtuoso in the art of scientific writing. It misses, though, part of the truth: that when it comes to sex—or history, or politics, or opera—science can answer all questions, except the interesting ones.

This Issue

July 17, 1997