(1) A friend of ours once told us about her rape. The details hardly matter, but in outline her story is numbingly familiar. After a movie she returned with her date to his car, which had been left in an isolated parking lot. She was expecting him to drive her home. Instead, the man locked the car doors and physically forced her to have sex with him.

(2) Our friend was emotionally scarred by her experience: she became anxious about dating, and even about going out in public. She had trouble sleeping, eating and concentrating on her work. Indeed, like some war veterans, rape victims often suffer from post-traumatic stress disorder, in which symptoms such as anxiety, memory loss, obsessive thoughts and emotional numbness linger after a deeply disturbing experience. Yet gruesome ordeals like that of our friend are all too common: in a 1992 survey of American women aged eighteen and older, 13 percent of the respondents reported having been the victim of at least one rape, where rape was defined as unwelcome oral, anal or vaginal penetration achieved through the use or threat of force. Surely, eradicating sexual violence is an issue that modern society should make a top priority. But first a perplexing question must be confronted and answered: Why do men rape?

(3) The quest for the answer to that question has occupied the two of us collectively for more than forty years. As a purely scientific puzzle, the problem is hard enough. But it is further roiled by strong ideological currents. Many social theorists view rape not only as an ugly crime but as a symptom of an unhealthy society, in which men fear and disrespect women. In 1975 the feminist writer Susan Brownmiller asserted that rape is motivated not by lust but by the urge to control and dominate. In the twenty-five years since, Brownmiller's view has become mainstream. All men feel sexual desire, the theory goes, but not all men rape. Rape is viewed as an unnatural behavior that has nothing to do with sex, and one that has no corollary in the animal world.

(4) Undoubtedly, individual rapists may have a variety of motivations. A man may rape because, for instance, he wants to impress his friends by losing his virginity, or because he wants to avenge himself against a woman who has spurned him. But social scientists have not convincingly demonstrated that rapists are not at least partly motivated by sexual desire as well. Indeed, how could a rape take place at all without sexual motivation on the part of the rapist? Isn't sexual arousal of the rapist the one common factor in all rapes, including date rapes, rapes of children, rapes of women under anesthetic and even gang rapes committed by soldiers during war?

(5) We want to challenge the dearly held idea that rape is not about sex. We realize that our approach and our frankness will rankle some social scientists, including some serious and well-intentioned rape investigators. But many facts point to the conclusion that rape is, in its very essence, a sexual act. Furthermore, we argue, rape has evolved over millennia of human history, along with courtship, sexual attraction and other behaviors related to the production of offspring.

(6) Consider the following facts:

• Most rape victims are women of childbearing age.
• In many cultures rape is treated as a crime against the victim's husband.
• Rape victims suffer less emotional distress when they are subjected to more violence.
• Rape takes place not only among human beings but also in a variety of other animal species.
• Married women and women of childbearing age experience more psychological distress after a rape than do girls, single women or women who are past menopause.

As bizarre as some of those facts may seem, they all make sense when rape is viewed as a natural, biological phenomenon that is a product of the human evolutionary heritage.

(7) Here we must hasten to emphasize that by categorizing a behavior as "natural" and "biological" we do not in any way mean to imply that the behavior is justified or even inevitable. Biological means "of or pertaining to life," so the word applies to every human feature and behavior. But to infer from that--as many of our critics assert that we do--that what is biological is somehow right or good, would be to fall into the so-called naturalistic fallacy. That mistake is obvious enough when one considers such natural disasters as epidemics, floods and tornadoes. In those cases it is clear that what is natural is not always desirable. And of course much can be, and is, done to protect people against natural threats--from administering antibiotics to drawing up emergency evacuation plans. In other words, the fact that rape is an ancient part of human nature in no way excuses the rapist

(8) Why, then, have the editors of scholarly journals refused to publish papers that treat rape from a Darwinian perspective? Why have pickets and audience protesters caused public lectures on the evolutionary basis of rape to be canceled or terminated? Why have investigators working to discover the evolutionary causes of rape been denied positions at universities?

(9) The reason is the deep schism between many social scientists and investigators such as ourselves who are proponents of what is variously called sociobiology or evolutionary psychology. Social scientists regard culture-- everything from eating habits to language--as an entirely human invention, one that develops arbitrarily. According to that view, the desires of men and women are learned behaviors. Rape takes place only when men learn to rape, and it can be eradicated simply by substituting new lessons. Sociobiologists, by contrast, emphasize that learned behavior, and indeed all culture, is the result of psychological adaptations that have evolved over long periods of time. Those adaptations, like all traits of individual human beings, have both genetic and environmental components. We fervently believe that, just as the leopard's spots and the giraffe's elongated neck are the result of aeons of past Darwinian selection, so also is rape.

(10) That conclusion has profound and immediate practical consequences. The rape-prevention measures that are being taught to police officers, lawyers, parents, college students and potential rapists are based on the prevailing social-science view, and are therefore doomed to fail. The Darwinian theory of evolution by natural selection is the most powerful scientific theory that applies to living things. As long as efforts to prevent rape remain uninformed by that theory, they will continue to be handicapped by ideas about human nature that are fundamentally inadequate. We believe that only by acknowledging the evolutionary roots of rape can prevention tactics be devised that really work.

(11) From a Darwinian perspective, every kind of animal--whether grasshopper or gorilla, German or Ghanaian--has evolved to produce healthy children that will survive to pass along their parents' genetic legacy. The mechanics of the phenomenon are simple: animals born without traits that led to reproduction died out, whereas the ones that reproduced the most succeeded in conveying their genes to posterity. Crudely speaking, sex feels good because over evolutionary time the animals that liked having sex created more offspring than the animals that didn't.

(12) As everyone knows all too well, however, sex and the social behaviors that go with it are endlessly complicated. Their mysterious and tangled permutations have inspired flights of literary genius throughout the ages, from Oedipus Rex to Portnoy's Complaint. And a quick perusal of the personal-growth section of any bookstore--past such titles as Men Are from Mars, Women Are from Venus and You Just Don't Understand--is enough to show that one reason sex is so complicated is that men and women perceive it so differently. Is that the case only because boys and girls receive different messages during their upbringing? Or, as we believe, do those differences between the sexes go deeper?

(13) Over vast periods of evolutionary time, men and women have confronted quite different reproductive challenges. Whereas fathers can share the responsibilities of child rearing, they do not have to. Like most of their male counterparts in the rest of the animal kingdom, human males can reproduce successfully with a minimal expenditure of time and energy; once the brief act of sexual intercourse is completed, their contribution can cease. By contrast, the minimum effort required for a woman to reproduce successfully includes nine months of pregnancy and a painful childbirth. Typically, ancestral females also had to devote themselves to prolonged breast-feeding and many years of child care if they were to ensure the survival of their genes. In short, a man can have many children, with little inconvenience to himself; a woman can have only a few, and with great effort.

(14) That difference is the key to understanding the origins of certain important adaptations--features that persist because they were favored by natural selection in the past. Given the low cost in time and energy that mating entails for the male, selection favored males who mated frequently. By contrast, selection favored females who gave careful consideration to their choice of a mate; that way, the high costs of mating for the female would be undertaken under circumstances that were most likely to produce healthy offspring. The result is that men show greater interest than women do in having a variety of sexual partners and in having casual sex without investment or commitment. That commonplace observation has been confirmed by many empirical studies. The evolutionary psychologist David M. Buss of the University of Texas at Austin, for instance, has found that women around the world use wealth, status and earning potential as major criteria in selecting a mate, and that they value those attributes in mates more than men do.

(15) Remember, none of the foregoing behavioral manifestations of evolution need be conscious. People do not necessarily have sex because they want children, and they certainly do not conduct thorough cost-benefit analyses before taking a partner to bed. As Darwin made clear, individual organisms merely serve as the instruments of evolution. Men today find young women attractive because during human evolutionary history the males who preferred prepubescent girls or women too old to conceive were outreproduced by the males who were drawn to females of high reproductive potential. And women today prefer successful men because the females who passed on the most genes, and thereby became our ancestors, were the ones who carefully selected partners who could best support their offspring. That is why, as the anthropologist Donald Symons of the University of California, Santa Barbara, has observed, people everywhere understand sex as "something females have that males want."

(16) A dozen roses, romantic dinners by candlelight, a Tiffany engagement ring: the classic courtship ritual requires lots of time, energy and careful attention to detail. But people are far from unique in that regard: the males of most animal species spend much of their energies attracting, wooing and securing sexual partners. The male woodcock, for instance, performs a dramatic display each spring at mating time, soaring high into the air and then tumbling to the ground. Male fireflies are even flashier, blinking like neon signs. The male bowerbird builds a veritable honeymoon cottage: an intricate, sculpted nest that he decorates with flowers and other colorful bric-a-brac. Male deer and antelope lock antlers in a display of brute strength to compete for females.

(17) Once a female's interest is piqued, the male behaves in various ways to make her more sexually receptive. Depending on the species, he dances, fans his feathers or offers gifts of food. In the nursery web spider, the food gift is an attempt to distract the female, who otherwise might literally devour her partner during the sex act. The common thread that binds nearly all animal species seems to be that males are willing to abandon all sense and decorum, even to risk their lives, in the frantic quest for sex.

(18) But though most male animals expend a great deal of time and energy enticing females, forced copulation--rape--also occurs, at least occasionally, in a variety of insects, birds, fishes, reptiles, amphibians, marine mammals and nonhuman primates. In some animal species, moreover, rape is commonplace. In many scorpionfly species, for instance--insects that one of us (Thornhill) has studied in depth--males have two well-formulated strategies for mating. Either they offer the female a nuptial gift (a mass of hardened saliva they have produced, or a dead insect) or they chase a female and take her by force.

(19) A remarkable feature of these scorpionflies is an appendage that seems specially designed for rape. Called the notal organ, it is a clamp on the top of the male's abdomen with which he can grab on to one of the female's forewings during mating, to prevent her escape. Besides rape, the notal organ does not appear to have any other function. For example, when the notal organs of males are experimentally covered with beeswax, to keep them from functioning, the males cannot rape. Such males still mate successfully, however, when they are allowed to present nuptial gifts to females. And other experiments have shown that the notal organ is not an adaptation for transferring sperm: in unforced mating, the organ contributes nothing to insemination.

(20) Not surprisingly, females prefer voluntary mating to mating by force: they will approach a male bearing a nuptial gift and flee a male that does not have one. Intriguingly, however, the males, too, seem to prefer a consensual arrangement: they rape only when they cannot obtain a nuptial gift. Experiments have shown that when male scorpionflies possessing nuptial gifts are removed from an area, giftless males--typically, the wimpier ones that had failed in male-male competitions over prey--quickly shift from attempting rape to guarding a gift that has been left untended. That preference for consensual sex makes sense in evolutionary terms, because when females are willing, males are much more likely to achieve penetration and sperm transfer.

(21) Human males obviously have no external organ specifically designed for rape. One must therefore look to the male psyche--to a potential mental rape organ--to discover any special-purpose adaptation of the human male to rape.

(22) Since women are choosy, men have been selected for finding a way to be chosen. One way to do that is to possess traits that women prefer: men with symmetrical body features are attractive to women, presumably because such features are a sign of health. A second way that men can gain access to women is by defeating other men in fights or other kinds of competitions--thereby gaining power, resources and social status, other qualities that women find attractive.

(23) Rape can be understood as a third kind of sexual strategy: one more way to gain access to females. There are several mechanisms by which such a strategy could function. For example, men might resort to rape when they are socially disenfranchised, and thus unable to gain access to women through looks, wealth or status. Alternatively, men could have evolved to practice rape when the costs seem low--when, for instance, a woman is alone and unprotected (and thus retaliation seems unlikely), or when they have physical control over a woman (and so cannot be injured by her). Over evolutionary time, some men may have succeeded in passing on their genes through rape, thus perpetuating the behavior. It is also possible, however, that rape evolved not as a reproductive strategy in itself but merely as a side effect of other adaptations, such as the strong male sex drive and the male desire to mate with a variety of women.

(24) Take, for instance, the fact that men are able to maintain sexual arousal and copulate with unwilling women. That ability invites inquiry, according to the psychologist Margo Wilson of McMaster University in Hamilton, Ontario, and her coworkers, because it is not a trait that is common to the males of all animal species. Its existence in human males could signal that they have evolved psychological mechanisms that specifically enable them to engage in forced copulation--in short, it could be a rape adaptation. But that is not the only plausible explanation. The psychologist Neil M. Malamuth of the University of California, Los Angeles, points out that the ability to copulate with unwilling women may be simply a by-product of men's "greater capacity for impersonal sex."

(25) More research is needed to decide the question of whether rape is an adaptation or merely a by-product of other sexual adaptations. Both hypotheses are plausible: one of us (Thornhill) supports the former, whereas the other (Palmer) endorses the latter. Regardless of which hypothesis prevails, however, there is no doubt that rape has evolutionary--and thus genetic--origins. All traits and behaviors stem from a complex interplay between genes and the environment. If rape is an adaptation, men must possess genes that exist specifically because rape increased reproductive success. If rape turns out to be merely a side effect of other adaptations, then the genes involved exist for reasons that have nothing to do with rape. Either way, however, the evolutionary perspective explains a number of otherwise puzzling facts about the persistence of rape among human males.

(26) For example, if rape is evolutionary in origin, it should be a threat mostly to women of childbearing age. And, in fact, young adult women are vastly overrepresented among rape victims in the female population as a whole, and female children and post-reproductive-age women are greatly underrepresented.

(27) By the same token, if rape has persisted in the human population through the action of sexual selection, rapists should not seriously injure their victims--the rapist's reproductive success would be hampered, after all, if he killed his victim or inflicted so much harm that the potential pregnancy was compromised. Once again, the evolutionary logic seems to predict reality. Rapists seldom engage in gratuitous violence; instead, they usually limit themselves to the force required to subdue or control their victims. A survey by one of us (Palmer), of volunteers at rape crisis centers, found that only 15 percent of the victims whom the volunteers had encountered reported having been beaten in excess of what was needed to accomplish the rape. And in a 1979 study of 1,401 rape victims, a team led by the sociologist Thomas W. McCahill found that most of the victims reported being pushed or held, but that acts of gratuitous violence, such as beating, slapping or choking, were reported in only a minority of the rapes--22 percent or less. A very small number of rape victims are murdered: about .01 percent (that figure includes unreported as well as reported rapes). Even in those few cases, it may be that the murder takes place not because the rapist is motivated by a desire to kill, but because by removing the only witness to the crime he greatly increases his chance of escaping punishment.

(28) Rape is more distressing for women than are other violent crimes, and evolutionary theory can help explain that as well. In recent years research on human unhappiness informed by evolutionary theory has developed substantial evidence about the functional role of psychological pain. Such pain is thought to be an adaptation that helps people guard against circumstances that reduce their reproductive success; it does so by spurring behavioral changes aimed at preventing future pain [see "What Good Is Feeling Bad?" by Randolph M. Nesse, November/December 1991]. Thus one would expect the greatest psychological pain to be associated with events that lower one's reproductive success, and, indeed, emotionally traumatic events such as the death of a relative, the loss of social status, desertion by one's mate and the trauma of being raped can all be interpreted as having that effect.

(29) Rape reduces female reproductive success in several ways. For one thing, the victim may be injured. Moreover, if she becomes pregnant, she is deprived of her chance to choose the best father for her children. A rape may also cause a woman to lose the investment of her long-term partner, because it calls into question whether the child she later bears is really his. A variety of studies have shown that both men and women care more for their genetic offspring than for stepchildren.

(30) One of us (Thornhill), in association with the anthropologist Nancy W. Thornhill, has conducted a series of studies on the factors that contribute to the emotional pain that women experience after a rape. Those studies confirmed that the more the rape interfered with the women's reproductive interests, the more pain they felt. The data, obtained from the Joseph J. Peters Institute in Philadelphia, came from interviews with 790 girls and women who had reported a sexual assault and who were subsequently examined at Philadelphia General Hospital between 1973 and 1975. The subjects, who ranged in age from two months to eighty-eight years, were asked a variety of questions designed to evaluate their psychological responses to the rape. Among other things, they were asked about changes in their sleeping habits, in their feelings toward known and unknown men, in their sexual relations with their partners (children were not asked about sexual matters), and in their eating habits and social activities.

(31) Analysis of the data showed that young women suffered greater distress after a rape than did children or women who were past reproductive age. That finding makes evolutionary sense, because it is young women who were at risk of being impregnated by an undesirable mate. Married women, moreover, were more traumatized than unmarried women, and they were more likely to feel that their future had been harmed by the rape. That, too, makes evolutionary sense, because the doubt a rape sows about paternity can lead a long-term mate to withdraw his support.

(32) Among the women in the study, psychological pain rose inversely to the violence of the attack. In other words, when the rapist exerted less force, the victim was more upset afterward. Those findings, surprising at first, make sense in the evolutionary context: a victim who exhibits physical evidence that sexual access was forced may have less difficulty convincing her husband or boyfriend that what took place was rape rather than consensual sex. In evolutionary terms, such evidence would be reassuring to a pair-bonded male, because rape is a one-time event, whereas consensual sex with other partners is likely to be frequent, and thus more threatening to paternity.

(33) Finally, women of reproductive age reported more emotional distress when the assault involved sexual intercourse than when it involved other kinds of sexual behavior. Among young girls and older women, however, penile-vaginal intercourse was no more upsetting than other kinds of assaults. Again, the possibility of an unwanted pregnancy may be a key factor in the degree of trauma the victim experiences.

(34) For all those reasons, the psychological pain that rape victims suffer appears to be an evolved defense against rape. The human females who outreproduced others--and thus became our ancestors--were people who were highly distressed about rape. Their distress presumably served their interests by motivating them to identify the circumstances that resulted in the rape, assess the problems the rape caused, and act to avoid rapes in the future.

(35) If women today are to protect themselves from rape, and men are to desist from it, people must be given advice that is based on knowledge. Insisting that rape is not about sex misinforms both men and women about the motivations behind rape--a dangerous error that not only hinders prevention efforts but may actually increase the incidence of rape.

(36) What we envision is an evolutionarily informed program for young men that teaches them to restrain their sexual behavior. Completion of such a course might be required, say, before a young man is granted a driver's license. The program might start by inducing the young men to acknowledge the power of their sexual impulses, and then explaining why human males have evolved in that way. The young men should learn that past Darwinian selection is the reason that a man can get an erection just by looking at a photo of a naked woman, why he may be tempted to demand sex even if he knows that his date truly doesn't want it, and why he may mistake a woman's friendly comment or tight blouse as an invitation to sex. Most of all, the program should stress that a man's evolved sexual desires offer him no excuse whatsoever for raping a woman, and that if he understands and resists those desires, he may be able to prevent their manifestation in sexually coercive behavior. The criminal penalties for rape should also be discussed in detail.

(37) Young women also need a new kind of education. For example, in today's rape-prevention handbooks, women are often told that sexual attractiveness does not influence rapists. That is emphatically not true. Because a woman is considered most attractive when her fertility is at its peak, from her mid-teens through her twenties, tactics that focus on protecting women in those age groups will be most effective in reducing the overall frequency of rape.

(38) Young women should be informed that, during the evolution of human sexuality, the existence of female choice has favored men who are quickly aroused by signals of a female's willingness to grant sexual access. Furthermore, women need to realize that, because selection favored males who had many mates, men tend to read signals of acceptance into a woman's actions even when no such signals are intended.

(39) In spite of protestations to the contrary, women should also be advised that the way they dress can put them at risk. In the past, most discussions of female appearance in the context of rape have, entirely unfairly, asserted that a victim's dress and behavior should affect the degree of punishment meted out to the rapist: thus if the victim was dressed provocatively, she "had it coming to her"--and the rapist would get off lightly. But current attempts to avoid blaming the victim have led to false propaganda that dress and behavior have little or no influence on a woman's chances of being raped. As a consequence, important knowledge about how to avoid dangerous circumstances is often suppressed. Sure-ly the point that no woman's behavior gives a man the right to rape her can be made with-out encouraging women to overlook the role they themselves may be playing in compromising their safety.

(40) Until relatively recently in Europe and the United States, strict social taboos kept young men and women from spending unsupervised time together, and in many other countries young women are still kept cloistered away from men. Such physical barriers are understandably abhorrent to many people, since they greatly limit the freedom of women. But the toppling of those barriers in modern Western countries raises problems of its own. The common practice of unsupervised dating in cars and private homes, which is often accompanied by the consumption of alcohol, has placed young women in environments that are conducive to rape to an extent that is probably unparalleled in history. After studying the data on the risk factors for rape, the sex investigators Elizabeth R. Allgeier and Albert R. Allgeier, both of Bowling Green State University in Ohio, recommended that men and women interact only in public places during the early stages of their relationships--or, at least, that women exert more control than they generally do over the circumstances in which they consent to be alone with men.

(41) An evolutionary perspective on rape might not only help prevent rapes but also lead to more effective counseling for rape victims. A therapy program explaining that men rape because they collectively want to dominate women will not help a victim understand why her attacker appeared to be sexually motivated, why she can no longer concentrate enough to conduct her life effectively, or why her husband or boyfriend may view the attack as an instance of infidelity. In addition, men who are made aware of the evolutionary reasons for their suspicions about their wives' or girlfriends' claims of rape should be in a better position to change their reactions to such claims.

(42) Unlike many other contentious social issues, such as abortion and homosexual rights, everyone has the same goal regarding rape: to end it. Evolutionary biology provides clear information that society can use to achieve that goal. Social science, by contrast, promotes erroneous solutions, because it fails to recognize that Darwinian selection has shaped not only human bodies but human psychology, learning patterns and behavior as well. The fact is that men, relative to women, are more aggressive, sexually assertive and eager to copulate, and less discriminating about mates--traits that contribute to the existence of rape. When social scientists mistakenly assert that socialization alone causes those gender differences, they ignore the fact that the same differences also exist in all the other animal species in which males offer less parental investment than females and compete for access to females.

(43) In addressing the question of rape, the choice between the politically constructed answers of social science and the evidentiary answers of evolutionary biology is essentially a choice between ideology and knowledge. As scientists who would like to see rape eradicated from human life, we sincerely hope that truth will prevail.

Randy Thornhill is an evolutionary biologist at the University of New Mexico in Albuquerque. Craig T. Palmer is an evolutionary anthropologist at the University of Colorado at Colorado Springs. This article was adapted from their forthcoming book A Natural History of Rape: Biological Bases of Sexual Coercion, which is being published in April 2000 by MIT Press.

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