Why women have better sex with rich men," said a recent headline in The Sunday Times announcing the latest research from Daniel Nettle, reader at Newcastle University's Centre for Behaviour and Evolution.
Nettle investigated the relationship between the frequency of Chinese women's self-reported orgasms and their partners' characteristics, using data from 1,534 women recorded in the Chinese Health and Family Life Survey.
He found that 121 women said they always had orgasms during sex, 408 had them "often", 762 "sometimes" and 243 rarely or never. Once other variables were controlled for, including women's age, health, education and happiness, the frequency of orgasms increased with the income of their partners.
Nettle acknowledged that the result could be "an artefact of reporting bias", but he said the findings could be interpreted to mean "more desirable mates cause women to experience more orgasms ... consistent with the view that female orgasm has an evolved, adaptive function".
"Many will object to the idea that women are hardwired to be gold-diggers," The Sunday Times article said. Steven Rose, professor of biology at The Open University, is among them. He describes the study as "frankly, an intellectual embarrassment that does no credit to the authors, referees or journal".
Self-reporting is notoriously unreliable, he points out. "And the results could be explained by all sorts of other reasons. For instance, the richer you are the more leisure time and domestic space you have, and the less likely you are to sleep with kids in the same room."
Nettle's findings are part of a burgeoning body of research under the banner of evolutionary psychology. This controversial approach, although less than 20 years old, has had a growing influence on most branches of psychology, as well as sociology, economics and political science.
It posits that the human mind evolved alongside the human body in the Stone Age to deal with environmental problems, and that some of our behavioural traits are evolved adaptations (see box page 44).
As one of the discipline's founders, Leda Cosmides, professor of psychology at the University of California, Santa Barbara, put it: "(It) is an approach to psychology in which knowledge and principles of evolutionary biology are put to use in research on the structure of the human mind."
But Rose argues that the theory's hypotheses are similar to Rudyard Kipling's Just So Stories and are incapable of being tested. Therefore, evolutionary psychology can be used - or exploited - to explain any response to a given situation and to argue that modern ideals and historical inequalities have adaptive origins.
"There's a lot of talk about how men have always preferred women with a high waist-hip ratio," he says. "Go to any great art gallery and look at the 17th- and 18th-century nudes, which you have to assume reflected the male sexual tastes of the day, and you'll see what nonsense that is."
The suggestion that women prefer rich men "is contradicted by social data showing that on the whole men and women choose to marry within their own class and cultural group", he adds. Darwinian theory has been "hijacked" by academics who are applying it unscientifically, Rose suggests.
Much research from evolutionary psychologists appears to be contradictory: in a study published in 2007, Meghan Provost in the department of psychology, Queen's University, Kingston, Canada, concluded that men found women's style of walking most attractive when they were least fertile.
She was reported as saying: "If women are trying to protect themselves from sexual assault at times of peak fertility, it would make sense for them to advertise attractiveness on a broad scale when they are not fertile."
On the other hand, Martie Haselton at the University of California Los Angeles department of psychology reported that women "dress to impress" when most fertile. "Women with high fertility tend to feel attracted to men other than their primary partners," said Dr Haselton.
Both studies were widely reported in the US and European national press, adding weight to Rose's claim that a lot of research using the tropes of Darwinian theory is of little scientific value but can be guaranteed to garner "a bit of vulgar publicity" for the author.
Other opponents of evolutionary psychology, including John Dupre, professor of the philosophy of science at the University of Exeter, argue that it is based on an outdated view of evolution. Neither our bodies nor our minds became "hard-wired" in the Pleistocene era, he maintains, but are perpetually evolving, with information from the environment constantly interacting with the genome.
Aubrey Sheiham, professor of dental public health at University College London, disagrees.
"We know that human nature is built around flexibility, but how does (Dupre) explain the instinct for language, or incest taboos?" he asks. "Nobody will deny that there are physical differences between the sexes, but talking about mental differences brings accusations of sexism.
"Why is autism more common in males? In Sweden, men can take the same amount of paternity leave as women, but they don't ... and if they do, they go fishing."
Sheiham believes that awareness of the evolutionary aspects of human behaviour will make public policy more effective.
Throughout this year, an Economic and Social Research Council-funded seminar series, Darwin's Medicine: Evolutionary Psychology and its Applications, aims to assess the discipline's value as a tool for making and informing public policy on matters as diverse as public health, business leadership and gang violence.
Sheiham is organising a seminar to discuss evolutionary approaches to poverty and health inequalities, including public policy on teenage pregnancy. He is critical of the Government's focus on education as a means of preventing early pregnancy. "It is giving girls health education and saying 'this is the disadvantage of having a baby'. What it is not grasping is that the psychology is saying: 'breed early because you won't live long'."
Areas with high levels of teenage pregnancy often have high levels of male teenage violence, too. One modern method of tackling the latter is by boosting boys' self-esteem.
"But if you're worried about teenage pregnancy, the worst thing you can do is increase self-esteem in the men, because they'll have more sex," Sheiham says. He concludes that many people are dismissive of evolutionary psychology because it threatens their political stance. "But if you want to change the world, you have to understand it."
Tom Dickins, principal lecturer in psychology at the University of East London, says evolutionary theory also helps to explain why differences exist within groups.
"They may have been observed before, but non-theoretically. We can now begin to make nuanced predictions about individual variation. Not all poor girls become teenage mothers; in fact, only a minority do."
As part of the Darwin's Medicine series, Dickins is organising a seminar on 5 April that will look at war, terrorism and hooliganism from the perspective of evolutionary psychology. This approach allows academics to look at violence at all levels between and including the state and the individual, he says.
Some studies have suggested that during war, crime rates at home fall if the population supports the need for the conflict.
"External threat leads to co-operation, and there is some discussion in the literature about group aggression having strategic value in our evolutionary past as a means of 'ramping up' our co-operative nature," Dickins says.
"We are an extremely co-operative species, which often gets overlooked in coverage of violence, and group-level aggression is an example of our ability to co-operate. This, I assume, is not a comfortable thought for policymakers."
While the debate about the credibility of the discipline continues, some politicians such as David Willetts, Shadow Secretary for Innovation, Universities and Skills, believe that theories on the evolution of reciprocity and fair behaviour may hold the key to a better society.
Mark van Vugt, professor of social psychology at the University of Kent and co-organiser of the seminar series, is studying the evolutionary origins of altruism.
"This was one of the themes that kept Charles Darwin awake at night," he says. "Why is one organism willing to promote the welfare of another at a cost to itself? It seems at odds with the theory of evolution by natural selection. The solution he came up with was that individual behaviour that benefits your group will help the group compete with other groups."
People help strangers because ancestral groups were small and made up mainly of our relatives, so our brains evolved to think that all people with whom we are in close proximity are relatives, he suggests.
While that might explain why people act altruistically towards neighbours, it does not shed much light on why they donate to causes that do not directly affect them.
"It may be that a reputation as a generous person can be valuable: people want to share resources with you or even pick you as a sexual partner," van Vugt says. In this light, charities should publicise the names of donors and the cash they donate as a technique to raise contributions.
"This would promote competitive altruism, where people compete to be seen as the most generous."
But this analysis is disputed by Rose, who questions why an evolutionary perspective on people's donating habits or public health is necessary. Observations of modern societies' behaviour should be sufficient to formulate policy, he suggests. "Why is there a need to drag Darwin into it?"
Robin Dunbar, professor of evolutionary anthropology at the University of Oxford, and one of the organisers of Darwin's Medicine, turns the question on its head and wonders why academics are so keen to exclude Darwin.
They have a "seemingly inevitable tendency" to reduce the evolutionary approach to nature versus nurture, he suggests. They then reject the "nature" argument because of their dislike for anything that smacks of genetic determinism.
Like Dupre, Dunbar believes that all aspects of human behaviour are the product of nature and nurture.
"Cultural evolution is as natural a component of the Darwinian world as something such as eye colour that is more obviously under genetic control," he says.
An artificial divide currently exists, he argues, between human behavioural ecologists, who study how ecological and social factors shape our actions, and some evolutionary psychologists who support the theory of "mind modules" - fixed cognitive processes in the brain that govern certain responses.
"Neither group is all wrong," Dunbar says. One of the aims of the seminar series is to "cover both wings of the discipline and bring them together".
"The key to making social change lies in figuring out how flexible people's behaviour is - pinpointing the bits that are open to adjustment and bearing in mind the parts that are resistant," he says.
For example, our social world is constrained by our cognitive ability, time and social pressures that dictate who we ought to talk to.
Dunbar's research has found that human social circles across many different societies and cultures are made up of about 150 individuals. His "social-brain" hypothesis postulates that the size of social groups characteristic of a species is a function of the size of the neocortex - the larger it is, the bigger the group.
The precise number varies depending on gender and individual differences in social cognitive skills.
"People who are better at juggling the mental states of more individuals at once (and women are better than men at this) have bigger social networks. And we now know that social cognitive skills are reflected in differences in how the brain works - which, of course, can be due to genetically inherited differences, differences in early nutrition, differences in rearing experiences or the interaction of all three."
Dunbar's view is that evolutionary theory allows academics to understand a simple phenomenon - such as how many friends we have - in terms of a range of different disciplines, including sociology, neurology, ecology and psychology.
"Our understanding of the phenomenon we began with is richer and more complete for this breadth of explanation," he adds.
"But more than that, our real appreciation of what is involved is possible only by integrating these disciplines into a single, seamless framework. The only framework we have that can do this job is evolutionary theory."
THOUGHTS, WORDS, DEEDS AND THE SWISS ARMY KNIFE
Evolutionary psychologists explore the application of Darwinian theory to human thought, feeling and action.
They generate hypotheses about behavioural responses and psychological traits, and then test them. They ask questions about the development of traits over geological time and the lifespan of individuals, how they enhance survival and reproduction, and which factors trigger certain behavioural responses.
The discipline aims to establish whether aspects of human behaviour are adaptations produced by natural selection, by-products of selection for other traits, or have no evolutionary function. Scientists have applied evolutionary theory to many aspects of human behaviour including mate selection, language and reciprocity.
The founders of evolutionary psychology, Leda Cosmides and John Tooby, have suggested that "mind modules" exist. These are cognitive processes developed to solve problems faced by our hunter-gatherer ancestors. Cosmides has compared the human mind to a Swiss Army knife - it has a large number of "programs" designed for solving different adaptive problems, such as choosing a good mate, caring for children and foraging for food.
This "mind module" theory is not universally accepted among evolutionary psychologists and has led critics of the discipline to accuse its proponents of genetic determinism.
But anthropologists such as Robin Dunbar say evolutionary psychology is concerned with how behavioural traits and responses serve the individual, and makes no specific assumptions about what determines them.
"It simply assumes that an individual's behaviour is guided by evolutionary considerations - that is, maximising its contribution to the species' gene pool," he says.
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