Rape is a mating strategy, say two US scientists. Ayala Ochert reports on the bitter debate started by Craig T. Palmer and Randy Thornhill, who argue, below, that rape is an act of sex, not violence

When two scientists announced that rape is a "natural adaptation", controversy was bound to follow. But with weeks still to go before the UK publication of Randy Thornhill and Craig Palmer's book, A Natural History of Rape: The Biological Bases of Sexual Coercion, there has already been an eruption of criticism. With an article by the two in the journal The Sciences, debate has exploded this month.

Thornhill and Palmer argue that rape evolved as an "alternative mating strategy", and they contend that it is a "natural, biological phenomenon and a product of our evolutionary heritage". They also take issue with the idea that rape is fundamentally an act of violence. That theory was put forward 25 years ago by feminist scholar Susan Brownmiller. In her treatise Against Our Will: Men, Women, and Rape, she described rape as an attempt by men to dominate and control women.

This week, Thornhill battled it out head to head with Brownmiller on US public radio, arguing that rape be regarded as an act of sex, not violence. Brownmiller was fiercely critical of the theory and of the scientists. "Obviously rape involves the sex organs, but what I was saying (in Against Our Will) was that rape is not sexy," Brownmiller said. "Men had romanticised rape and saw it as a Robin Hood act of machismo. But for women it is not sexy - it is pure humiliation and degradation."

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Thornhill and Palmer consistently state that their theory does not justify rape and argue that their aim is to eradicate the behaviour, but they have nonetheless encountered vehement opposition wherever they have gone. Scheduled lectures have been cancelled. And their argument that "just because something is 'natural' does not make it right" has not been persuasive to many who fear that rapists may try to use it as a defence in court.

It was Thornhill's work on the mating habits of the scorpion fly that led him to think that rape might be an evolutionary adaptation in our own species. Male scorpion flies have a "notal organ" - a kind of clamp whose only known function is to immobilise female flies for forced mating. Thornhill now concludes that a "specialised rape organ" exists in the human male psyche. He suggests that staying sexually aroused during non-consensual sex is an adaptation that conferred a selective advantage on some of our ancestors. Although rape is not common in the animal kingdom, they say, it has been observed in several species, including the orang-utan.

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Brownmiller persuaded a generation that sexual attraction is not important in rape by arguing that rape is a crime of violence. Thornhill and Palmer suggest otherwise. They point out that most rape victims are young women of child-bearing age, and they believe that rapists are sexually attracted to their victims. Brownmiller counters that a sizeable minority - as much as 22 per cent - of rape victims are children.

Criticism of Thornhill and Palmer's theory has come not just from the social scientists but also from those sympathetic to the idea that evolution has shaped human behaviour. In a retort to The Sciences article, Time magazine essayist Barbara Ehrenreich expresses doubts that rape would offer much of a selective advantage: "The rapist generally operates on a hit-and-run basis - which may be all right for stocking sperm banks, but is not quite so effective if the goal is to produce offspring who will survive in a challenging environment. The children of guys who raped and ran must have been a scrawny lot and doomed to end up on some leopard's lunch menu."

Evolutionary biologist Jerry Coyne of the University of Chicago has called the book "the worst efflorescence of evolutionary psychology that I have ever seen", claiming that it is "irresponsible, tendentious and the science is sloppy". Rape, he says, is pathological, not natural.

Almost as contentious as their theory of why men rape has been Thornhill and Palmer's recommendations for how to prevent it. They have drawn fire for their suggestion that young women be advised of the dangers of plunging necklines and short skirts.

"There is no evidence that scanty dress induces rape," Coyne says. As for Thornhill and Palmer's recommendation that young men be educated about the evolutionary basis of rape, Mary Koss, professor of public health at the University of Arizona and an authority on rape, says: "(They) have obviously never stood up before a group and given a rape-prevention talk. If you even imply to a male audience that all men are potential rapists, They go berserk!" THE EVOLUTIONARY BASIS OF RAPE

What does evolution have to do with rape? Before you answer nothing at all, please consider. Rape is unquestionably a heinous human behaviour, but it is still one performed by biological - ie living - organisms. And as the eminent biologist Theodore Dobzhansky observed more than 25 years ago: "Nothing in biology makes sense, except in the light of evolution." In order to prevent rape - one of the most abhorrent behaviours men engage in - it is crucial to understand its causes as fully as possible.

All aspects of living things, including morphology, physiology, cognition and behaviour, result from a complex interaction of genes and environmental factors. Genes alone do not determine any aspect of living things, and neither do environmental factors. Indeed, the interaction of genes and environment is so intertwined that no aspect of a living thing can be accurately depicted as either primarily genetic or environmental.

The evolutionary basis of the gene-environment interactions that produce rape is almost certainly linked to the different obstacles to reproduction faced by our male and female ancestors. Because of women's greater investment of time and energy in each potential child, a key to female reproductive success was mating with, and only with, the highest quality men available.

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To produce a child, men needed to make a much smaller investment of time and energy, so a key to male reproductive success was mating with multiple females.

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These differing selective pressures led to numerous different adaptations in the bodies, including the brains, of contemporary men and women. These include differences in sexual desire. Men have a greater interest in multiple sexual partners and in impersonal sex, and a greater willingness to mate without commitment. Rape, we suggest, is one of the many behaviours resulting from these evolved differences in male and female sexuality.

The next question is whether rape is an adaptation or a by-product of these evolved differences. Adaptations are those traits that exist in contemporary humans because natural selection favoured them in our evolutionary past. Adaptations are solutions to obstacles to human survival and reproduction. By-products are traits that exist as side-effects of other adaptations.

Much of evolutionary biological research is directed towards determining whether particular traits are adaptations or by-products. In the case of adaptations, the goal is to identify the evolutionary function of the trait by determining its functional design and thereby the kind of selection pressures that created that trait during evolutionary history. In the case of by-products, the goal is to identify the adaptations that produce the trait as a side-effect. In both instances, this line of research can generate insights into the immediate factors that produce the trait. Hence, it can provide new knowledge about how certain highly undesirable traits such as rape can be prevented by manipulating their immediate causes.

In some species in which rape occurs, it appears to be an adaptation. Male scorpion flies have a clamp-like organ that seems to be designed specifically to allow the male to copulate with unwilling females. Given that humans have no such external body part, the question of adaptation will probably be settled by identifying specific psychological mechanisms. There may be mechanisms that are designed for rape rather than for consensual sexual acts, such as men's sexual arousal patterns and sperm production and the physiological responses involved in their victim selection.

The popular perspective on rape, the social science theory, posits that rape is caused primarily or only by "culture", or social learning, which is presented as a quasi-metaphysical force that determines human behaviour. But, in fact, culture is totally biological - learning from members of one's own species, like all learning, occurs within the living brains of living beings and is guided by learning adaptations. The relevant learning adaptations involved in human rape appear to be possessed by men only and focus on valuing rape outside their immediate social groups. The ubiquity of widespread rape in warfare and in inter-village raiding supports our view that this learning is not arbitrary.

The social science model also advocates that rape is caused by men's desire to violently dominate women rather than by sexual motivation. We provide numerous data-based arguments as to why rape arises from men's evolved sexual motivations. Vast research reveals that sexual arousal is the universal common denominator in all rapes whether perpetrated on children, dates, wives, strangers or other victims. The levels of violence and force involved in rape typically stop at the level needed for the goal of sexual access. Gratuitous violence is not typical of rape. The social science model has also emphasised that rape is restricted to humans, but this ignores the fact that rape is widespread across all animal groups.

Our suggestions for preventing rape include education courses for both men and women about the evolution of human sexuality and warnings to women, especially to young women - who are the primary victims of rape - of the risk factors associated with western dating and dress. We have found in teaching thousands of university students that there is great interest in acquiring scientific knowledge about rape.

The findings of science have nothing to say about what is morally right or wrong. People are in agreement that rape is wrong and desire its extinction. Where science - specifically evolutionary biology - can help, is in providing salient knowledge for achieving this goal. The identification of the evolutionary basis of a trait implies nothing about the moral rightness of the trait. To think otherwise is to commit the "naturalistic fallacy", a fallacy that some critics of our work continue to make despite having the fallacious logic of their position explained many times by evolutionary scientists.

Neither does the identification of the evolutionary basis of a trait imply anything about the inevitability of the trait, a point many recent critics of our work also fail to comprehend. Instead, identification of the specific ways that genes interact with the environment to produce a behaviour can increase our ability to change the environment in ways that will decrease the chances of the behaviour taking place.

To ignore the knowledge that evolutionary biology can provide on why men rape women is the morally questionable stance.

Randy Thornhill is Regents' professor and professor of biology at the University of New Mexico. Craig Palmer is instructor of anthropology at the University of Colorado. A Natural History of Rape: The Biological Bases of Sexual Coercion, MIT Press, March 1.

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