Biologically, however, that moment of pleasure, the orgasmic pay-off, is not just a sensory bribe. In women, at least, it seems to be an important factor in the scramble among sperm to fertilise the egg. Recent evidence suggests that the timing of female orgasm relative to the male partner's ejaculation may influence sperm retention and allow the woman to manipulate the chances of the sperm from a particular male fertilising her egg. Orgasms thus become an important means of exercising mating preferences. If this is the case, the achievement and timing of female orgasm should reflect the desirability of the male partner. Studies by Randy Thornhill and his colleagues at the University of New Mexico suggest that it does, and that what makes a male a real earth-mover is the evenness of his features.
Evolutionary biologists have recently discovered that a form of bodily asymmetry may be an important factor in the mating preferences of several species. Human beings are bilaterally symmetrical - the left-hand side of our bodies is, at least outwardly, a mirror image of the right-hand side. But this symmetry is not exact. There are, for example, left-right differences in the size of hands, feet, ankles and ears. This is an example of what is known as fluctuating asymmetry because it varies randomly within populations.
In bilaterally symmetrical species, females seem to prefer males whose paired features show little difference between one side of their body and the other. There is a good biological reason for this. Fluctuating asymmetry appears to be a symptom of disturbance during early stages of the male's physical development, perhaps as a result of nutritional stress, disease or simply low genetic quality. If susceptibility to these problems is heritable, lop-sided males will make a poor choice of father for the female's offspring.
Evidence from Thornhill and co-workers' studies suggests that asymmetry in humans (here measured as left-right differences in hand, foot, wrist, ankle and ear size) has a marked effect on male mating success. Not only are relatively symmetrical men more likely to be judged attractive, but they also have more sexual partners and sexual relationships outside their long-term pair bond, and achieve first copulation with a new partner after shorter periods of courtship. Most recently, in research just published in the scientific journal Animal Behaviour, Thornhill's group has found that the tendency for women to orgasm during intercourse is influenced by the asymmetry of their male partner.
Even when other features of male and female partners and difference in the nature of individual relationships were taken into account, women in Thornhill's study turned out to have more orgasms during intercourse when their partners showed low fluctuating asymmetry. The fact that asymmetry affected orgasm during copulation, rather than foreplay, is important. Earlier work by Robin Baker and Mark Bellis has shown that orgasms at this time tend to result in a high degree of sperm retention, possibly through a muscular "upsuck" effect. Orgasms during foreplay tend to increase loss of sperm through later flowback from the vagina.
Few other features of men predicted female copulatory orgasm, although facial attractiveness and body weight had some effect independently of asymmetry. The upshot thus seems to be that copulatory orgasm in women is designed to retain the sperm of men with a good developmental pedigree, and also those that are facially attractive and large. Facial attractiveness and body weight both correlate positively with symmetry in a number of studies, so it may be that they act as easy indices of the developmental quality of the male. In a species in which females frequently mate with more than one male and pitch their sperm into a competitive play-off, orgasm seems to be a fun way of loading the dice in the female's favour.
The writer is in the Behaviour and Ecology Research Group at Nottingham University.