Is generosity just showing off?

Ever wondered why men usually pay the bill on a first date? An evolutionary psychologist might point out that men on dates also tend to leave unusually large tips and that gives us an important clue.

More specifically, two related evolutionary theories could give us the answer. The first, sexual selection, predicts that because females have a greater investment in parenting, they tend to prefer males who seem likely to provide resources and commitment. Males, meanwhile, have co-evolved to behave so as to give every appearance of being able to provide and care for their partner.

The problem is that, where behaviour is concerned, appearances can be deceptive. Is he really so wealthy and caring? Costly signalling theory, however, predicts that costliness is a reliable indicator of honest behaviours. The more it costs a man to appear capable and committed, for example, the more likely that he really is.

These theories might explain why male chimpanzees use food-sharing to show off to sexually receptive females. But do they really tell us anything about the way we behave? Wendy Iredale, at the University of Kent, and her colleagues decided to investigate by looking at charity donations in the presence of attractive members of the opposite sex.

They asked men and women (heterosexual undergraduates specifically) to play a series of games in which they could earn up to £24. They were then given the option of making a costly act of generosity - donating a percentage of their earnings to charity - either in private or observed by an attractive man or woman.

The results, published last week in the journal Evolutionary Psychology, showed that men are substantially more charitable when observed by the attractive female. As predicted by evolutionary theory, women's donations did not vary. However, scientists still don't know what exactly this generosity signals (caring or wealth, for example) or whether women actually find altruistic men more attractive.

Sexual selection also predicts that males evolve a preference for mates who appear to be fertile and, correspondingly, that females will co-evolve attributes that reliably signal their fertility. An example is breast size and symmetry which, so the argument goes, are honest indicators of fertility. And in case anyone needs convincing that men tend to respond to this particular adaptation, Nicolas Guegen from the Universite de Bretagne Sud studied drivers passing an attractive female hitch-hiker. The results, published in Perceptual and Motor Skills last December, showed that men are more likely to stop if her bra cup size appears to be C rather than A. Women stopped equally frequently in either case.

So what does all this tell us? Obviously, we've also evolved the intelligence and capacity for self-reflection to rise above these kinds of thing to a large extent. They don't fundamentally limit the decisions we make and nor do they excuse poor behaviour. But it's interesting to see residual influences of our evolutionary past still cropping up and being used to test general theories of behavioural evolution. Does anyone have any other examples?