This summer The É«ºÐÖ±²¥S will be exploring the intellectual impact of the new Darwinism. Here, Aisling Irwin describes how these ideas are having an effect beyond biology, while overleaf Brian Goodwin and Linda Partridge debate the significant issues within the discipline itself. Later articles will look at the influence of evolutionary ideas on other subjects.

It was when Chris Knight clicked to his next slide, an artist's impression of a hunter-gatherer family complete with topless mother, that the heckling started. The audience shouted "exploitative" and then shouted "sexual stereotyping" when he suggested that such mothers needed their male companions as food providers. But it was when he used the word "menstruation" (for he studies its social function) that they stormed out. Well, some stormed out: for every jeer there were at least two cheers, says Knight, an anthropologist at the University of East London.

Cheers or jeers, his work is provocative because he is harnessing the ideas of evolutionary biology - "selfish gene" theory - to explain how human rituals arose. To do it, he is straddling the alienated worlds of social anthropology and sociobiology. Social anthropology has debunked the notion that there is a universal human nature, while sociobiology depends on it. And sociobiology has long been associated with a right-wing agenda of legitimising human inequality.

Ever since Charles Darwin proposed that living things evolve by the fittest surviving to pass on their superior qualities to the next generation, thinkers have found the possibility of harnessing biological evolution to explain how human society arose irresistible. First they gather data to show that there is a universal human nature; second they claim that these universal characteristics are coded in our genes; third they claim that the reason we have these particular genes rather than others is that natural selection - the environment favouring the survival and reproduction of some genes over others - has inevitably led to these characteristics. Thus, genes are responsible for how human societies are.

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This enterprise has been regarded with distaste all century because of its implication: that human nature is completely fixed. Sociobiology has been used to justify hatred of strangers, male sexual dominance and warfare - because they are human universals. But such claims have recently been swept aside by a new wave of interest in Darwin's ideas. Academics from many disciplines gathered at a É«ºÐÖ±²¥S conference in 1993 and they are meeting at Darwin seminars at the London School of Economics over the summer. Instead of making grand claims to explain the whole of human culture they are inching forward, trying to support all they do with empirical data. Instead of pursuing, or allowing themselves to be manipulated by, a right-wing agenda, many claim that a leftist agenda of political change results from their work.

"I wouldn't be surprised to see, in five years, that this movement is allied to a leftist political position," says Randolph Nesse, an associate professor at Michigan University in Ann Arbor. "But we have been very, very careful not to allow people to box us into any political position."

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Nesse is a psychiatrist. He has pioneered the use of evolutionary ideas as a way of understanding psychiatric problems. Instead of regarding certain mental states as abnormalities, he thinks they could be adaptations that somehow confer an advantage in the struggle for survival. They may enhance social success, which is important for improving reproductive success. Consider, for example, self-deception. Psychotherapy tries to overcome it: but it may in fact be extremely useful for maintaining good relationships - or for manipulating people.

Nesse says he has met no opposition to his ideas from psychiatrists. There is more controversy over the ideas of David Buss, an evolutionary psychologist from the University of Michigan in Detroit, who specialises in explaining the differences between male and female behaviour. In his recent book, The Evolution of Desire, he shows how, in order to maximise their reproductive success, males need to adopt very different strategies from females. This leads to conflict between males and females as their strategies clash, and competition within each sex for members of the opposite sex. Crudely put, males maximise the passing on of their genes by increasing their number of offspring and females do so by gathering the resources to ensure that their offspring live to reproduce.

From a more detailed working out of male and female needs, Buss made predictions about their behaviour. He then gathered data. Across 37 cultures, he says, he found that women valued financial prospects in men more than men did in women, even if the women themselves were already wealthy; in every culture men preferred younger mates while women prefer older mates. And the single best predictor of the occupational status of a man was the physical attractiveness of his wife.

Buss has also tried physiological tests: he wires up a man, tells him to imagine his long-term partner having sex with someone else and measures the degree to which the man frowns, sweats and palpitates. Then he tells him to imagine his wife falling in love with someone else and measures him again. He finds that men are more upset by the sexual image than the emotional one; women respond the other way round.

Buss says his ideas are accepted within psychology. Outside it, a straw poll of leading left-wing biologists and anthropologists yields: "crude", "offensive", "outrageous", "crap", "awful". This is followed by a tumble of counter examples from other cultures. Tim Ingold, an anthropologist at Manchester University and critic of sociobiology, says: "The whole way in which these generalisations are cast takes no account of the details of culture." He objects to the implication that men and women are programmed and are not free to make choices.

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But Buss says that this objection stems from a common misunderstanding about the level at which he claims there is universality of human nature. "There is a difference between underlying mechanisms, which are difficult to change, and their expression as behaviour. Behaviour is very changeable." He denies that the work leads to a politics of unchangeability: "Among evolutionary psychologists you see a variety of political views from left wing socialist all the way to right wing and libertarian. Most of the key figures in evolutionary psychology are just scientists who are not driven by a political agenda." And, he says: "If there's one domain that you would have expected evolution to have sculpted it's mine: reproduction".

If evolution has sculpted our capacity for language, for altruism or for co-operation, can we do anything about them? A key stage in the evolution of society was the emergence of co-operation, which is being investigated by Leda Cosmides, a psychologist at the University of California, Santa Barbara. There have been many suggestions about how a complex, co-operative society could have grown up through a series of stages, each of which gave the individual adaptive advantages. But Cosmides has gone further. She has proposed the evolution of an instinct for reasoning, a special brain pathway, which humans developed so they could spot cheats in a social contract system heavily dependent on everyone pulling their weight. To support her argument she has used well-known tests of reasoning powers. People do much better at spotting cheats (75 per cent success rate versus 25 per cent) if, in a particular test, cheating would affect their own welfare.

Cosmides thinks we are locked into Pleistocene minds with adaptations that are not necessarily suitable for today. So is the new sociobiology a science of empowerment and flexibility or of social fixity? The answer, say critics of extreme genetic determinism, lies in how you invoke brain structure to explain the new findings. Buss thinks that there are specialised neural circuits which deal with mate selection or food selection. He says that empirical support for such specific modules is trickling in from the cutting edge of cognitive neuroscience.

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Robert Hinde, professor of biology at Cambridge University, says: "I agree with most of Buss's conclusions but he wants to explain them in terms of a large number of modules in the brain, and I feel that there are rather fewer, more plastic modules".

Steven Rose, a professor of biology at the Open University, says: "It's a debate about specificity versus plasticity". Specificity is the idea that our behaviour is hard-wired, in the same way that the eyes are hard-wired to the brain, he says. Plasticity means that our brain structure has been selected for its flexibility - because it enables us to respond to a variety of situations." Both explanations of our behaviour come from genetics.

Robin Dunbar, professor of psychology at Liverpool University, says that many evolutionary psychologists have not understood the flexible nature of evolutionary biology. "Darwinian psychologists are locked into these biologically underwritten social processes. But evolutionary biology is about the adaptation and fine adjustment of behaviour. It is variability of behaviour that enables evolution to occur." From this point of view, it is flexibility that survives, and it is the environment that determines what the optimal behaviour is.

Dunbar says: "The social scientists feel that anything to do with biology means that you can't change it. We know now that that isn't so. Change is the root of the whole thing. If you change the circumstances then you change the organism. All you have to do to change people's behaviour is to change the costs and benefits of what they do. Then the left-wing programme becomes feasible. In fact, a lot of the implications of this programme are very politically correct".

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