It is not often that science provides a good excuse to buy pornography. Yet students of the likely social consequences of the credit crunch may wish to keep a close eye on the pages of Playboy over the coming months.

According to Terry Pettijohn and Brian Jungeberg, of Mercyhurst College in Pennsylvania, the curves of the magazine's famous Playmates change in line with prosperity. Their study of centrefolds from 1960-2000 found that during booms models tended to be young, short and wasp-waisted, with wide hips and ample breasts. When times were hard, models were older and taller, with bigger waist-to-hip ratios, shallower curves and less body fat. Busts, if you like, are bad for busts.

These trends led the scientists to propose the “environmental security hypothesis”. It holds that in our evolutionary past, straitened circumstances led men away from the WAG-like figures they prefer when food is plentiful. Instead, they went for stronger, more experienced, more athletic women, who could muck in with the hunting and gathering.

The “Playboy effect” is a modern manifestation of Stone Age minds.

Such reasoning is a staple of evolutionary psychology, the subject of what is certain to be a fierce debate next Sunday at the Battle of Ideas, supported by Body&Soul.

This young science seeks to explain human behaviour today with reference to mental adaptations that served our species well long ago. It holds that just as physical features that enhanced survival and the ability to have offspring were passed on preferentially to future generations, so, too, were psychological traits.

Evolutionary psychology is often advanced to shed light on the roots of all manner of social phenomena, from the differing sexual behaviour of men and women, to altruism, crime and morality. It may explain, for instance, why children with a stepfather face a higher risk of murder.

They are unrelated to him and might compete for resources with his genetic children. Murder could thus enhance the prospects of the stepfather's genes.

On one level, the idea that human behaviour is influenced by evolution should be uncontroversial. The brain, just as certainly as the thumb, is an evolved organ. But the claim is often seen as incendiary.

First, it seems to countervail free will and the notion that people can be moulded by experience and culture.

What is more, it can appear politically incorrect, reinforcing gender stereotypes of race and gender. There is concern that to interpret rape or murder in terms of evolutionary advantage is tantamount to excusing them. And work such as the step-family research is understandably resented by the overwhelming majority of loving stepfathers.

Evolutionary psychologists are also apt to make observations and then to speculate wildly about potentially adaptive explanations. A good example is a study that suggested that girls' preference for pink is rooted in women's ancient role in gathering berries. A nice idea, but a prime case of what Stephen Jay Gould condemned as “Just-So Stories”.

It is certainly true that much evolutionary psychology lacks evidence and that some practitioners have been guilty of sloppy conjecture. Yet it can and does make testable predictions - the backbone of science - some of which are confirmed by observation in multiple cultures. The idea that men, on average, prefer youth and beauty in a partner, while women prefer status, is now quite well attested.

Both detractors and supporters of evolutionary psychology are prone to misjudging and exaggerating its claims. It does not propose that every move we make is genetically hard-wired, merely that propensities that promoted survival in the past are not irrelevant to behaviour today. It explores broad themes, such as why males tend to find females of a particular age and shape attractive. It says nothing about the contingent reasons why any individual man fancies any individual woman.

Neither is the existence of evolved human impulses incompatible with free will, or with environmental influences. Predispositions can be overcome: that rape might sometimes have reproductive advantages does not make it right. And hypotheses such as the Playboy effect have a role for both nature and nurture: the whole point is that genetic programmes are activated by environmental cues. This fits well with recent discoveries about how genes interact with experience to affect traits such as intelligence, aggression and mental health.

Evolutionary psychology is easy to caricature, but that does not mean our mental evolution has no influence today. When its limits are acknowledged, it is a backdrop we need to understand to make better sense of the many other factors that affect the human condition.

Mark Henderson is the science editor of The Times. His book, 50 Genetics Ideas You Really Need To Know, will be published in April by Quercus

FREE TICKETS FOR THE BATTLE OF IDEAS

The Battle of Ideas is a two- day festival on November 1-2 that celebrates open and lively debate. Body&Soul columnists Mark Henderson, Suzi Godson, Amanda Ursell and Martyn Lobley will be appearing, and we are offering readers the chance to win tickets - ten pairs are up for grabs, worth £100 each. To enter, e-mail Body&Soul@thetimes.co.uk, with your name, address and preferred day, by midnight on Sunday. battleofideas.org.uk

Terms and conditions 1. Draw open to residents of the UK and ROI aged 18 or over except employees of The Times their families, agents or any other person connected with the draw. 3. One entry per person. 4. Winners will be drawn at random from all eligible entries and will be notified by e-mail within 3 days of the closing date. 5.Only one pair of tickets per winner. 6. All other costs and expenses are the responsibility of the winners.

Explore Health

• Expert Advice

• Health Features

• Mental Health

• Alternative Medicine

• Child Health

• Health Club