Male And Female - Sex, Romance, And Love

The Moral Animal - Whe We Are The Way We Are: The New Science of Evolutionary Psychology - Robert Wright 1995

Male And Female
Sex, Romance, And Love

In the most distinct classes of the animal kingdom, with mammals, birds, reptiles, fishes, insects, and even crustaceans, the differences between the sexes follow almost exactly the same rules; the males are almost always the wooers....

The Descent of Man (1871)1

Darwin was wrong about sex.

He wasn't wrong about the males being the wooers. His reading of the basic characters of the two sexes holds up well today. "The female, ... with the rarest exception, is less eager than the male.... [S]he is coy, and may often be seen endeavouring for a long time to escape from the male. Every one who has attended to the habits of animals will be able to call to mind instances of this kind... . The exertion of some choice on the part of the female seems almost as general a law as the eagerness of the male."2

Nor was Darwin wrong about the consequences of this asymmetrical interest. He saw that female reticence left males competing with one another for scarce reproductive opportunities, and thus explained why males so often have built-in weapons — the horns of stags, the hornlike mandibles of stag beetles, the fierce canines of chimpanzees.3 Males not hereditarily equipped for combat with other {33} males have been excluded from sex, and their traits have thus been discarded by natural selection.

Darwin also saw that the choosiness of females gives great moment to their choices. If they prefer to mate with particular kinds of males, those kinds will proliferate. Hence the ornamentation of so many male animals — a lizard's inflatable throat sack, brightly colored during the mating season; the immense, cumbersome tail of the peacock; and, again, the stag's horns, which seem more elaborate than the needs of combat alone would dictate.4 These decorations have evolved not because they aid in daily survival — if anything, they complicate it — but because they can so charm a female as to outweigh the everyday burdens they bring. (How it came to be in the genetic interest of females to be charmed by such things is another story, and a point of subtle disagreement among biologists.)5

Both of these variants of natural selection — combat among males and discernment by females — Darwin called "sexual selection." He took great pride in the idea, and justifiably so. Sexual selection is a nonobvious extension of his general theory that accounts for seeming exceptions to it (like garish colors that virtually say "Kill me" to predators), and that has not just endured over time but grown in scope.

What Darwin was wrong about was the evolutionary cause of female coyness and male eagerness. He saw that this imbalance of interest creates competition among males for precious reproductive slots, and he saw the consequences of this competition; but he didn't see what had created the imbalance. His attempt late in life to explain the phenomenon was unsuccessful.6 And, in fairness to him, whole generations of biologists would do no better.

Now that there is a consensus on the solution, the long failure to find it seems puzzling. It's a very simple solution. In this sense, sex is typical of many behaviors illuminated by natural selection; though the illumination has gotten truly powerful only within the last three decades, it could in principle have done so a century earlier, so plainly does it follow from Darwin's view of life. There is some subtle logic involved, so Darwin can be forgiven for not having seen the full scope of his theory. Still, if he were around today to hear evolutionary biologists talk about sex, he might well sink into one {34} of his self-effacing funks, exclaiming at his obtuseness in not getting the picture sooner.


The first step toward understanding the basic imbalance of the sexes is to assume hypothetically the role natural selection plays in designing a species. Take the human species, for example. Suppose you're in charge of installing, in the minds of human (or prehuman) beings, rules of behavior that will guide them through life, the object of the game being to maximize each person's genetic legacy. To oversimplify a bit: you're supposed to make each person behave in such a way that he or she is likely to have lots of offspring — offspring, moreover, who themselves have lots of offspring.

Obviously, this isn't the way natural selection actually works. It doesn't consciously design organisms. It doesn't consciously do anything. It blindly preserves hereditary traits that happen to enhance survival and reproduction.* Still, natural selection works as if it were consciously designing organisms, so pretending you're in charge of organism design is a legitimate way to figure out which tendencies evolution is likely to have ingrained in people and other animals. In fact, this is what evolutionary biologists spend a good deal of time doing: looking at a trait — mental or otherwise — and figuring out what, if any, engineering challenge it is a solution to.

When playing the Administrator of Evolution, and trying to maximize genetic legacy, you quickly discover that this goal implies different tendencies for men and women. Men can reproduce hundreds of times a year, assuming they can persuade enough women to cooperate, and assuming there aren't any laws against polygamy — which there assuredly weren't in the environment where much of our evolution took place. Women, on the other hand, can't reproduce more often than once a year. The asymmetry lies partly in the high price of eggs; in all species they're bigger and rarer than minuscule, {35} mass-produced sperm. (That, in fact, is biology's official definition of a female: the one with the larger sex cells.) But the asymmetry is exaggerated by the details of mammalian reproduction; the egg's lengthy conversion into an organism happens inside the female, and she can't handle many projects at once.

So, while there are various reasons why it could make Darwinian sense for a woman to mate with more than one man (maybe the first man was infertile, for example), there comes a time when having more sex just isn't worth the trouble. Better to get some rest or grab a bite to eat. For a man, unless he's really on the brink of collapse or starvation, that time never comes. Each new partner offers a very real chance to get more genes into the next generation — a much more valuable prospect, in the Darwinian calculus, than a nap or a meal. As the evolutionary psychologists Martin Daly and Margo Wilson have succinctly put it: for males "there is always the possibility of doing better."7

There's a sense in which a female can do better, too, but it has to do with quality, not quantity. Giving birth to a child involves a huge commitment of time, not to mention energy, and nature has put a low ceiling on how many such enterprises she can undertake. So each child, from her (genetic) point of view, is an extremely precious gene machine. Its ability to survive and then, in turn, produce its own young gene machines is of mammoth importance. It makes Darwinian sense, then, for a woman to be selective about the man who is going to help her build each gene machine. She should size up an aspiring partner before letting him in on the investment, asking herself what he'll bring to the project. This question then entails a number of subquestions that, in the human species especially, are more numerous and subtle than you might guess.

Before we go into these questions, a couple of points must be made. One is that the woman needn't literally ask them, or even be aware of them. Much of the relevant history of our species took place before our ancestors were smart enough to ask much of anything. And even in the more recent past, after the arrival of language and self-awareness, there has been no reason for every evolved behavioral tendency to fall under conscious control. In fact, sometimes it is emphatically not in our genetic interest to be aware of exactly what {36} we are doing or why. (Hence Freud, who was definitely onto something, though some evolutionary psychologists would say he didn't know exactly what.) In the case of sexual attraction, at any rate, everyday experience suggests that natural selection has wielded its influence largely via the emotional spigots that turn on and off such feelings as tentative attraction, fierce passion, and swoon-inducing infatuation. A woman doesn't typically size up a man and think: "He seems like a worthy contributor to my genetic legacy." She just sizes him up and feels attracted to him — or doesn't. All the "thinking" has been done — unconsciously, metaphorically — by natural selection. Genes leading to attractions that wound up being good for her ancestors' genetic legacies have flourished, and those leading to less productive attractions have not.

Understanding the often unconscious nature of genetic control is the first step toward understanding that — in many realms, not just sex — we're all puppets, and our best hope for even partial liberation is to try to decipher the logic of the puppeteer. The full scope of the logic will take some time to explain, but I don't think I'm spoiling the end of the movie by noting here that the puppeteer seems to have exactly zero regard for the happiness of the puppets.

The second point to grasp before pondering how natural selection has "decided" to shape the sexual preferences of women (and of men) is that it isn't foresightful. Evolution is guided by the environment in which it takes place, and environments change. Natural selection had no way of anticipating, for example, that someday people would use contraception, and that their passions would thus lead them into time-consuming and energy-sapping sex that was sure to be fruitless; or that X-rated videotapes would come along and lead indiscriminately lustful men to spend leisure time watching them rather than pursuing real, live women who might get their genes to the next generation.

This isn't to say that there's anything wrong with "unproductive" sexual recreation. Just because natural selection created us doesn't mean we have to slavishly follow its peculiar agenda. (If anything, we might be tempted to spite it for all the ridiculous baggage it's saddled us with.) The point is just that it isn't correct to say that people's minds are designed to maximize their fitness, their genetic {37} legacy. What the theory of natural selection says, rather, is that people's minds were designed to maximize fitness in the environment in which those minds evolved. This environment is known as the EEA — the environment of evolutionary adaptation.8 Or, more memorably: the "ancestral environment." Throughout this book, the ancestral environment will lurk in the background. At times, in pondering whether some mental trait is an evolutionary adaptation, I will ask whether it seems to be in the "genetic interest" of its bearer. For example: Would indiscriminate lust be in the genetic interest of men? But this is just a kind of shorthand. The question, properly put, is always whether a trait would be in the "genetic interest" of someone in the EEA, not in modern America or Victorian England or anywhere else. Only traits that would have propelled the genes responsible for them through the generations in our ancestral social environment should, in theory, be part of human nature today.9

What was the ancestral environment like? The closest thing to a twentieth-century example is a hunter-gatherer society, such as the !Kung San of the Kalahari Desert in Africa, the Inuit (Eskimos) of the Arctic region, or the Ache of Paraguay. Inconveniently, hunter-gatherer societies are quite different from one another, rendering simple generalization about the crucible of human evolution difficult. This diversity is a reminder that the idea of a single EEA is actually a fiction, a composite drawing; our ancestral social environment no doubt changed much in the course of human evolution.10 Still, there are recurring themes among contemporary hunter-gatherer societies, and they suggest that some features probably stayed fairly constant during much of the evolution of the human mind. For example: people grew up near close kin in small villages where everyone knew everyone else and strangers didn't show up very often. People got married — monogamously or polygamously — and a female typically was married by the time she was old enough to be fertile.

This much, at any rate, is a safe bet: whatever the ancestral environment was like, it wasn't much like the environment we're in now. We aren't designed to stand on crowded subway platforms, or to live in suburbs next door to people we never talk to, or to get hired or fired, or to watch the evening news. This disjunction between the contexts of our design and of our lives is probably responsible {38} for much psychopathology, as well as much suffering of a less dramatic sort. (And, like the importance of unconscious motivation, it is an observation for which Freud gets some credit; it is central to his Civilization and Its Discontents.)

To figure out what women are inclined to seek in a man, and vice versa, we'll need to think more carefully about our ancestral social environment(s). And, as we'll see, thinking about the ancestral environment also helps explain why females in our species are less sexually reserved than females in many other species. But for purposes of making the single, largest point of this chapter — that, whatever the typical level of reserve for females in our species, it is higher than the level for males — the particular environment doesn't much matter. For this point depends only on the premise that an individual female can, over a lifetime, have many fewer offspring than an individual male. And that has been the case, basically, forever: since before our ancestors were human, before they were primates, before they were mammals — way, way back through the evolution of our brain, down to its reptilian core. Female snakes may not be very smart, but they're smart enough to know, unconsciously, at least, that there are some males it's not a good idea to mate with.

Darwin's failure, then, was a failure to see what a deeply precious commodity females are. He saw that their coyness had made them precious, but he didn't see that they were inherently precious — precious by virtue of their biological role in reproduction, and the resulting slow rate of female reproduction. Natural selection had seen this — or, at least, had "seen" it — and female coyness is the result of this implicit comprehension.


The first large and clear step toward human comprehension of this logic was made in 1948 by the British geneticist A. J. Bateman. Bateman took fruit flies and ran them through a dating game. He would place five males and five females in a chamber, let them follow their hearts, and then, by examining the traits of the next generation, figure out which offspring belonged to which parents. He found a clear pattern. Whereas almost all females had about the same number of offspring, regardless of whether they mated with one, two, or {39} three males, male legacies differed according to a simple rule: the more females you mate with, the more offspring you have. Bateman saw the import: natural selection encourages "an undiscriminating eagerness in the males and a discriminating passivity in the females."11

Bateman's insight long lay essentially unappreciated. It took nearly three decades, and several evolutionary biologists, to give it the things it lacked: full and rigorous elaboration on the one hand, and publicity on the other.

The first part — the rigor — came from two biologists who are good examples of how erroneous some stereotypes about Darwinism are. In the 1970s, opposition to sociobiology often took the form of charges that its practitioners were closet reactionaries, racists, fascists, etcetera. It is hard to imagine two people less vulnerable to such charges than George Williams and Robert Trivers, and it is hard to name anyone who did more than they to lay the foundations of the new paradigm.

Williams, a professor emeritus at the State University of New York, has worked hard to dispel vestiges of social Darwinism, with its underlying assumption that natural selection is a process somehow worthy of obedience or emulation. Many biologists share his view, and stress that we can't derive our moral values from its "values." But Williams goes further. Natural selection, he says, is an "evil" process, so great is the pain and death it thrives on, so deep is the selfishness it engenders.

Trivers, who was an untenured professor at Harvard when the new paradigm was taking shape and is now at Rutgers University, is much less inclined than Williams toward moral philosophy. But he evinces an emphatic failure to buy the right-wing values associated with social Darwinism. He speaks proudly of his friendship with the late Black Panther leader Huey Newton (with whom he once co-authored an article on human psychology). He rails against the bias of the judicial system. He sees conservative conspiracies where some people don't.

In 1966 Williams published his landmark work, Adaptation and Natural Selection: A Critique of Some Current Evolutionary Thought. {40}

Slowly this book has acquired a nearly holy stature in its field. It is a basic text for biologists who think about social behavior, including human social behavior, in light of the new Darwinism.12 Williams's book dispelled confusions that had long plagued the study of social behavior, and it laid down foundational insights that would support whole edifices of work on the subjects of friendship and sex. In both cases Trivers would be instrumental in building the edifices.

Williams amplified and extended the logic behind Bateman's 1948 paper. He cast the issue of male versus female genetic interests in terms of the "sacrifice" required for reproduction. For a male mammal, the necessary sacrifice is close to zero. His "essential role may end with copulation, which involves a negligible expenditure of energy and materials on his part, and only a momentary lapse of attention from matters of direct concern to his safety and well-being." With little to lose and much to gain, males can profit, in the currency of natural selection, by harboring "an aggressive and immediate willingness to mate with as many females as may be available." For the female, on the other hand, "copulation may mean a commitment to a prolonged burden, in both the mechanical and physiological sense, and its many attendant stresses and dangers." Thus, it is in her genetic interest to "assume the burdens of reproduction" only when circumstances seem propitious. And "one of the most important circumstances is the inseminating male"; since "unusually fit fathers tend to have unusually fit offspring," it is "to the female's advantage to be able to pick the most fit male available...."13

Hence courtship: "the advertisement, by a male, of how fit he is." And just as "it is to his advantage to pretend to be highly fit whether he is or not," it is to the female's advantage to spot false advertising. So natural selection creates "a skilled salesmanship among the males and an equally well-developed sales resistance and discrimination among the females."14 In other words: males should, in theory, tend to be show-offs.

A few years later, Trivers used the ideas of Bateman and Williams to create a full-blown theory that ever since has been shedding light on the psychology of men and women. Trivers began by replacing Williams's concept of "sacrifice" with "investment." The difference {41} may seem slight, but nuances can start intellectual avalanches, and this one did. The term investment, linked to economics, comes with a ready-made analytical framework.

In a now-famous paper published in 1972, Trivers formally defined "parental investment" as "any investment by the parent in an individual offspring that increases the offspring's chance of surviving (and hence [the offspring's] reproductive success) at the cost of the parent's ability to invest in other offspring."15 Parental investment includes the time and energy consumed in producing an egg or a sperm, achieving fertilization, gestating or incubating the egg, and rearing the offspring. Plainly, females will generally make the higher investment up until birth, and, less plainly but in fact typically, this disparity continues after birth.

By quantifying the imbalance of investment between mother and father in a given species, Trivers suggested, we could better understand many things — for example, the extent of male eagerness and female coyness, the intensity of sexual selection, and many subtle aspects of courtship and parenthood, fidelity and infidelity. Trivers saw that in our species the imbalance of investment is not as stark as in many others. And he correctly suspected that (as we'll see in the next chapter) the result is much psychological complexity.

At last, with Trivers's paper "Parental Investment and Sexual Selection," the flower had bloomed; a simple extension of Darwin's theory — so simple that Darwin would have grasped it in a minute — had been glimpsed in 1948, clearly articulated in 1966, and was now, in 1972, given full form.16 Still, the concept of parental investment lacked one thing: publicity. It was E. O. Wilson's book Sociobiology (1975) and Richard Dawkins's The Selfish Gene (1976) that gave Trivers's work a large and diverse audience, getting scores of psychologists and anthropologists to think about human sexuality in modern Darwinian terms. The resulting insights are likely to keep accumulating for a long time.


Theories are a dime a dozen. Even strikingly elegant theories, which, like the theory of parental investment, seem able to explain much {42} with little, often turn out to be worthless. There is justice in the complaint (from creationists, among others) that some theories about the evolution of animal traits are "just so stories" — plausible, but nothing more. Still, it is possible to separate the merely plausible from the compelling. In some sciences, testing theories is so straightforward that it is only a slight exaggeration (though it is always, in a certain strict sense, an exaggeration) to talk of theories being "proved." In others, corroboration is roundabout — an ongoing, gradual process by which confidence approaches the threshold of consensus, or fails to. Studying the evolutionary roots of human nature, or of anything else, is a science of the second sort. About each theory we ask a series of questions, and the answers nourish belief or doubt or ambivalence.

One question about the theory of parental investment is whether human behavior in fact complies with it in even the most basic ways. Are women more choosy about sex partners than men? (This is not to be confused with the very different question, to which we'll return, of which sex, if either, is choosier about marriage partners.) Certainly there is plenty of folk wisdom suggesting as much. More concretely, there's the fact that prostitution — sex with someone you don't know and don't care to know — is a service sought overwhelmingly by males, now as in Victorian England. Similarly, virtually all pornography that relies sheerly on visual stimulation — pictures or films of anonymous people, spiritless flesh — is consumed by males. And various studies have shown men to be, on average, much more open to casual, anonymous sex than women. In one experiment, three fourths of the men approached by an unknown woman on a college campus agreed to have sex with her, whereas none of the women approached by an unknown man were willing.17

It used to be common for doubters to complain that this sort of evidence, drawn from Western society, reflects only its warped values. This tack has been problematic since 1979, when Donald Symons published The Evolution of Human Sexuality, the first comprehensive anthropological survey of human sexual behavior from the new Darwinian perspective. Drawing on cultures East and West, industrial and preliterate, Symons demonstrated the great breadth of the {43} patterns implied by the theory of parental investment: women tend to be relatively selective about sex partners; men tend to be less so, and tend to find sex with a wide variety of partners an extraordinarily appealing concept.

One culture Symons discussed is about as far from Western influence as possible: the indigenous culture of the Trobriand Islands in Melanesia. The prehistoric migration that populated these islands broke off from the migrations that peopled Europe at least tens of thousands of years ago, and possibly more than 100,000 years ago. The Trobrianders' ancestral culture was separated from Europe's ancestral culture even earlier than was that of Native Americans.18 And indeed, when visited by the great anthropologist Bronislaw Malinowski in 1915, the islands proved startlingly remote from the currents of Western thought. The natives, it seemed, hadn't even gotten the connection between sex and reproduction. When one seafaring Trobriander returned from a voyage of several years to find his wife with two children, Malinowski was tactful enough not to suggest that she had been unfaithful. And "when I discussed the matter with others, suggesting that one at least of these children could not be his, my interlocutors did not understand what I meant."19

Some anthropologists have doubted that the Trobrianders could have been so ignorant. And although Malinowski's account of this issue seems to have the ring of authority, there is no way of knowing whether he got the story straight. But it is important to understand that he could, in principle, be right. The evolution of human sexual psychology seems to have preceded the discovery by humans of what sex is for. Lust and other such feelings are natural selection's way of getting us to act as if we wanted lots of offspring and knew how to get them, whether or not we actually do.20 Had natural selection not worked this way — had it instead harnessed human intelligence so that our pursuit of fitness was entirely conscious and calculated — then life would be very different. Husbands and wives would, for example, spend no time having extramarital affairs with contraception; they would either scrap the contraception or scrap the sex.

Another un-Western thing about Trobriand culture was the lack of Victorian anxiety about premarital sex. By early adolescence, both girls and boys were encouraged to mate with a series of partners to {44} their liking. (This freedom is found in some other preindustrial societies, though the experimentation typically ends, and marriage begins, before a girl reaches fertility.) But Malinowski left no doubt about which sex was choosier. "[TJhere is nothing roundabout in a Trobriand wooing... . Simply and directly a meeting is asked for with the avowed intention of sexual gratification. If the invitation is accepted, the satisfaction of the boy's desire eliminates the romantic frame of mind, the craving for the unattainable and mysterious. If he is rejected, there is not much room for personal tragedy, for he is accustomed from childhood to having his sexual impulses thwarted by some girl, and he knows that another intrigue cures this type of ill surely and swiftly. ..." And: "In the course of every love affair the man has constantly to give small presents to the woman. To the natives the need of one-sided payment is self-evident. This custom implies that sexual intercourse, even where there is mutual attachment, is a service rendered by the female to the male."21

There were certainly cultural forces reinforcing coyness among Trobriand women. Though a young woman was encouraged to have an active sex life, her advances would be frowned on if too overt and common because of the "small sense of personal worth that such urgent solicitation implies."22 But is there any reason to believe this norm was anything other than a culturally mediated reflection of deeper genetic logic? Can anyone find a single culture in which women with unrestrained sexual appetites aren't viewed as more aberrant than comparably libidinous men? If not, isn't it an astonishing coincidence that all peoples have independently arrived at roughly the same cultural destination, with no genetic encouragement? Or is it the case that this universal cultural element was present half a million or more years ago, before the species began splitting up? That seems a long time for an essentially arbitrary value to endure, without being extinguished in a single culture.

This exercise holds a couple of important lessons. First: one good reason to suspect an evolutionary explanation for something — some mental trait or mechanism of mental development — is that it's universal, found everywhere, even in cultures that are as far apart as two cultures can be.23 Second: the general difficulty of explaining such universality in utterly cultural terms is an example of how the {45} Darwinian view, though not proved right in the sense that mathematical theorems are proved right, can still be the view that, by the rules of science, wins; its chain of explanation is shorter than the alternative chain and has fewer dubious links; it is a simpler and more potent theory. If we accept even the three meager assertions made so far — (1) that the theory of natural selection straightforwardly implies the "fitness" of women who are choosy about sexual partners and of men who often aren't; (2) that this choosiness and unchoosiness, respectively, is observed worldwide; and (3) that this universality can't be explained with equal simplicity by a competing, purely cultural, theory — if we accept these things, and if we're playing by the rules of science, we have to endorse the Darwinian explanation: male license and (relative) female reserve are to some extent innate.

Still, it is always good to have more evidence. Though absolute "proof" may not be possible in science, varying degrees of confidence are. And while evolutionary explanations rarely attain the 99.99 percent confidence sometimes found in physics or chemistry, it's always nice to raise the level from, say, 70 to 97 percent.

One way to strengthen an evolutionary explanation is to show that its logic is obeyed generally. If women are choosy about sex because they can have fewer kids than men (by virtue of investing more in them), and if females in the animal kingdom generally can have fewer offspring than males, then female animals in general should be choosier than males. Evolutionary theories can generate falsifiable predictions, as good scientific theories are expected to do, even though evolutionary biologists don't have the luxury of rerunning evolution in their labs, with some of its variables controlled, and predicting the outcome.

This particular prediction has been abundantly confirmed. In species after species, females are coy and males are not. Indeed, males are so dim in their sexual discernment that they may pursue things other than females. Among some kinds of frogs, mistaken homosexual courtship is so common that a "release call" is used by males who find themselves in the clutches of another male to notify him that they're both wasting their time.24 Male snakes, for their part, have been known to spend a while with dead females before moving on to a live prospect.25 And male turkeys will avidly court a stuffed {46} replica of a female turkey. In fact, a replica of a female turkey's head suspended fifteen inches from the ground will generally do the trick. The male circles the head, does its ritual displays, and then (confident, presumably, that its performance has been impressive) rises into the air and comes down in the proximity of the female's backside, which turns out not to exist. The more virile males will show such interest even when a wooden head is used, and a few can summon lust for a wooden head with no eyes or beak.26

Of course, such experiments only confirm in vivid form what Darwin had much earlier said was obvious: males are very eager. This raises a much-cited problem with testing evolutionary explanations: the odd sense in which a theory's "predictions" are confirmed. Darwin didn't sit in his study and say, "My theory implies coy, picky females and mindlessly lustful males," and then take a walk to see if he could find examples. On the contrary, the many examples are what prompted him to wonder which implication of natural selection had created them — a question not correctly answered until midway through the following century, after even more examples had piled up. This tendency for Darwinian "predictions" to come after their evident fulfillment has been a chronic gripe of Darwin's critics. People who doubt the theory of natural selection, or just resist its application to human behavior, complain about the retrofitting of fresh predictions to preexisting results. This is often what they have in mind when they say evolutionary biologists spend their time dreaming up "just so stories" to explain everything they see.

In a sense, dreaming up plausible stories is what evolutionary biologists do. But that's not by itself a damning indictment. The power of a theory, such as the theory of parental investment, is gauged by how much data it explains and how simply, regardless of when the data surfaced. After Copernicus showed that assuming the Earth to revolve around the Sun accounted elegantly for the otherwise perplexing patterns that stars trace in the sky, it would have been beside the point to say, "But you cheated. You knew about the patterns all along." Some "just so stories" are plainly better than others, and they win. Besides, how much choice do evolutionary biologists have? There's not much they can do about the fact that the database on animal life began accumulating millennia before Darwin's theory. {47}

But there is one thing they can do. Often a Darwinian theory generates, in addition to the pseudopredictions that the theory was in fact designed to explain, additional predictions — real predictions, untested predictions, which can be used to further evaluate the theory. (Darwin elliptically outlined this method in 1838, at age twenty-nine — more than twenty years before The Origin of Species was published. He wrote in his notebook: "The line of argument pursued throughout my theory is to establish a point as a probability by induction, & to apply it as hypothesis to other points, & see whether it will solve them.")27 The theory of parental investment is a good example. For there are a few oddball species, as Williams noted in 1966, in which the male's investment in the offspring roughly matches, or even exceeds, the female's. If the theory of parental investment is right, these species should defy sex stereotypes.

Consider the spindly creatures known as pipefish. Here the male plays a role like a female kangaroo's: he takes the eggs into a pouch and plugs them into his bloodstream for nutrition. The female can thus start on another round of reproduction while the male is playing nurse. This may not mean that she can have many more offspring than he in the long run — after all, it took her a while to produce the eggs in the first place. Still, the parental investment isn't grossly imbalanced in the usual direction. And, predictably, female pipefish tend to take an active role in courtship, seeking out the males and initiating the mating ritual.28

Some birds, such as the phalarope (including the two species known as sea snipes), exhibit a similarly abnormal distribution of parental investment. The males sit on the eggs, leaving the females free to go get some wild oats sown. Again, we see the expected departure from stereotype. It is the phalarope females who are larger and more colorful — a sign that sexual selection is working in reverse, as females compete for males. One biologist observed that the females, in classically male fashion, "quarrel and display among themselves" while the males patiently incubate the eggs.29

If the truth be told, Williams knew that these species defy stereotype when he wrote in 1966. But subsequent investigation has confirmed his "prediction" more broadly. Extensive parental investment by males has been shown to have the expected consequences {48} in other birds, in the Panamanian poison-arrow frog, in a water bug whose males cart fertilized eggs around on their backs, and in the (ironically named, it turns out) Mormon cricket. So far Williams's prediction has encountered no serious trouble.30


There is another major form of evolutionary evidence bearing on differences between men and women: our nearest relatives. The great apes — chimpanzees, pygmy chimps (also called bonobos), gorillas, and orangutans — are not, of course, our ancestors; all have evolved since their path diverged from ours. Still, those forks in the road have come between eight million years ago (for chimps and bonobos) and sixteen million years ago (for orangutans).31 That's not long, as these things go. (A reference point: The australopithecine, our presumed ancestor, whose skull was ape-sized but who walked upright, appeared between six and four million years ago, shortly after the chimpanzee off-ramp. Homo erectus — the species that had brains midway in size between ours and apes', and used them to discover fire — took shape around 1.5 million years ago.)32

The great apes' nearness to us on the evolutionary tree legitimizes a kind of detective game. It's possible — though hardly certain — that when a trait is shared by all of them and by us, the reason is common descent. In other words: the trait existed in our common, sixteen-million-year-old proto-ape ancestor, and has been in all our lineages ever since. The logic is roughly the same as tracking down four distant cousins, finding that they all have brown eyes, and inferring that at least one of their two common great-great-grandparents had brown eyes. It's far from being an airtight conclusion, but it has more credence than it had when you had seen only one of the cousins.33

Lots of traits are shared by us and the great apes. For many of the traits — five-fingered hands, say — pointing this out isn't worth the trouble; no one doubts the genetic basis of human hands anyway. But in the case of human mental traits whose genetic substratum is still debated — such as the differing sexual appetites of men and women — this inter-ape comparison can be useful. Besides, it's worth taking a minute to get acquainted with our nearest relatives. Who {49} knows how much of our psyche we share by common descent with some or all of them?

Orangutan males are drifters. They wander in solitude, looking for females, who tend to be stationary, each in her own home range. A male may settle down long enough to monopolize one, two, or even more of these ranges, though vast monopolies are discouraged by the attendant need to fend off vast numbers of rivals. Once the mission is accomplished, and the resident female gives birth, the male is likely to disappear. He may return years later, when pregnancy is again possible.34 In the meantime, he doesn't bother to write.

For a gorilla male, the goal is to become leader of a pack comprising several adult females, their young offspring, and maybe a few young adult males. As dominant male, he will get sole sexual access to the females; the young males generally mind their manners (though a leader may, as he ages and his strength ebbs, share females with them).35 On the downside, the leader does have to confront any male interlopers, each of which aims to make off with one or more of his females and thus is in an assertive mood.

The life of the male chimpanzee is also combative. He strives to climb a male hierarchy that is long and fluid compared to a gorilla hierarchy. And, again, the dominant male — working tirelessly to protect his rank through assault, intimidation, and cunning — gets first dibs on any females, a prerogative he enforces with special vigor when they're ovulating.36

Pygmy chimps, or bonobos (they're actually a distinct species from chimpanzees), may be the most erotic of all primates. Their sex comes in many forms and often serves purposes other than reproduction. Periodic homosexual behavior, such as genital rubbings between females, seems to be a way of saying, "Let's be friends." Still, broadly speaking, the bonobos' sociosexual outline mirrors that of the common chimpanzees: a pronounced male hierarchy that helps determine access to females.37

Amid the great variety of social structure in these species, the basic theme of this chapter stands out, at least in minimal form: males seem very eager for sex and work hard to find it; females work less hard. This isn't to say the females don't like sex. They love it, and may initiate it. And, intriguingly, the females of the species most {50} closely related to humans — chimpanzees and bonobos — seem particularly amenable to a wild sex life, including a variety of partners. Still, female apes don't do what male apes do: search high and low, risking life and limb, to find sex, and to find as much of it, with as many different partners, as possible; it has a way of finding them.


That female apes are, on balance, more reticent than male apes doesn't necessarily mean that they actively screen their prospective partners. To be sure, the partners get screened; those who dominate other males mate, while those who get dominated may not. This competition is exactly what Darwin had in mind in defining one of the two kinds of sexual selection, and these species (like our own) illustrate how it favors the evolution of big, mean males. But what about the other kind of sexual selection? Does the female participate in the screening, choosing the male that seems the most auspicious contributor to her project?

Female choice is notoriously hard to spot, and signs of its long-term effect are often ambiguous. Are males larger and stronger than females just because tougher males have bested their rivals and gotten to mate? Or, in addition, have females come to prefer tough males, since females with this genetically ingrained preference have had tougher and therefore more prolific sons, whose many daughters then inherited their grandmother's taste?

Notwithstanding such difficulties, it's fairly safe to say that in one sense or another, females are choosy in all the great ape species. A female gorilla, for example, though generally confined to sex with a single, dominant male, normally emigrates in the course of her lifetime. When an alien male approaches her pack, engaging its leader in mutual threats and maybe even a fight, she will, if sufficiently impressed, decide to follow him.38

In the case of chimps, the matter is more subtle. The dominant, or alpha, male can have any female he wants, but that's not necessarily because she prefers him; he shuts off alternatives by frightening other males. He can frighten her too, so that any spurning of low-ranking males may reflect only her fear. (Indeed, the spurning has been known {51} to disappear when the alpha isn't looking.)39 But there is a wholly different kind of chimp mating — a sustained, private consortship that may be a prototype for human courtship. A male and female chimp will leave the community for days or even weeks. And although the female may be forcibly abducted if she resists an invitation, there are times when she successfully resists, and times when she chooses to go peacefully, even though nearby males would gladly aid her in any resistance.40

Actually, even going unpeacefully can involve a kind of choice. Female orangutans are a good example. They do often seem to exercise positive choice, favoring some males over others. But sometimes they resist a mating and are forcibly subdued and — insofar as this word can be applied to nonhumans — raped. There is evidence that the rapists, often adolescents, usually fail to impregnate.41 But suppose that they succeed with some regularity. Then a female, in sheerly Darwinian terms, is better off mating with a good rapist, a big, strong, sexually aggressive male; her male offspring will then be more likely to be big, strong, and sexually aggressive (assuming sexual aggressiveness varies at least in part because of genetic differences) — and therefore prolific. So female resistance should be favored by natural selection as a way to avoid having a son who is an inept rapist (assuming it doesn't bring injury to the female).

This isn't to say that a female primate, her protests notwithstanding, "really wants it," as human males have been known to assume. On the contrary, the more an orangutan "really wants it," the less she'll resist, and the less powerful a screening device her reticence will be. What natural selection "wants" and what any individual wants needn't be the same, and in this case they're somewhat at odds. The point is simply that, even when females demonstrate no clear preference for certain kinds of males, they may be, in practical terms, preferring a certain kind of male. And this de facto discretion may be de jure. It may be an adaptation, favored by natural selection precisely because it has this filtering effect.

In the broadest sense, the same logic could apply in any primate species. Once females in general begin putting up the slightest resistance, then a female that puts up a little extra resistance is exhibiting a valuable trait. For whatever it takes to penetrate resistance, the sons {52} of strong resisters are more likely to have it than the sons of weak resisters. (This assumes, again, that the relative possession by different males of "whatever it takes" reflects underlying genetic differences.) Thus, in sheerly Darwinian terms, coyness becomes its own reward. And this is true regardless of whether the male's means of approach is physical or verbal.


A common reaction to the new Darwinian view of sex is that it makes perfect sense as an explanation for animal behavior — which is to say, for the behavior of nonhuman animals. People may chuckle appreciatively at a male turkey that tries to mate with a poor rendition of a female's head, but if you then point out that many a human male regularly gets aroused after looking at two-dimensional representations of a nude woman, they don't see the connection. After all, the man surely knows that it's only a photo he's looking at; his behavior may be pathetic, but it isn't comic.

And maybe it isn't. But if he "knows" it's a photo, why is he getting so excited? And why are women so seldom whipped up into an onanistic frenzy by pictures of men?

Resistance to lumping humans and turkeys under one set of Darwinian rules has its points. Yes, our behavior is under more subtle, presumably more "conscious," control than is turkey behavior. Men can decide not to get aroused by something — or, at least, can decide not to look at something they know will arouse them. Sometimes they even stick with those decisions. And although turkeys can make what look like comparable "choices" (a turkey hounded by a shotgun-wielding man may decide that now isn't the time for romance), it is plainly true that the complexity and subtlety of options available to a human are unrivaled in the animal kingdom. So too is the human's considered pursuit of very long-run goals.

It all feels very rational, and in some ways it is. But that doesn't mean it isn't in the service of Darwinian ends. To a layperson, it may seem natural that the evolution of reflective, self-conscious brains would liberate us from the base dictates of our evolutionary past. To an evolutionary biologist, what seems natural is roughly the opposite: that human brains evolved not to insulate us from the mandate to {53} survive and reproduce, but to follow it more effectively, if more pliably; that as we evolve from a species whose males forcibly abduct females into a species whose males whisper sweet nothings, the whispering will be governed by the same logic as the abduction — it is a means of manipulating females to male ends, and its form serves this function. The basic emanations of natural selection are refracted from the older, inner parts of our brain all the way out to its freshest tissue. Indeed, the freshest tissue would never have appeared if it didn't toe natural selection's bottom line.

Of course, a lot has happened since our ancestors parted ways with the great apes' ancestors, and one can imagine a change in evolutionary context that would have removed our lineage from the logic that so imbalances the romantic interests of male and female in most species. Don't forget about the seahorses, sea snipes, Panamanian poison-arrow frogs, and Mormon crickets, with their reversed sex roles. And, less dramatically, but a bit closer to home, there are the gibbons, another of our primate cousins, whose ancestors waved good-bye to ours about twenty million years ago. At some point in gibbon evolution, circumstances began to encourage much male parental investment. The males regularly stick around and help provide for the kids. In one gibbon species the males actually carry the infants, something male apes aren't exactly known for. And talk about marital harmony: gibbon couples sing a loud duet in the morning, pointedly advertising their familial stability for the information of would-be home-wreckers.42

Well, human males too have been known to carry around infants, and to stay with their families. Is it possible that at some time over the last few million years something happened to us rather like what happened to the gibbons? Have male and female sexual appetites converged at least enough to make monogamous marriage a reasonable goal? {54}