RECEIVED wisdom and biological theory both have it that males are (or, at least, would like to be) more promiscuous than females. That does not stop a lot of men settling down happily as faithful husbands. Conversely, wisdom and theory also suggest that once a woman has kissed the frog who turns into a prince, she will stick with him till death do them part. But that is belied by the number of females who wander from man to man, or simply do without a helpmeet altogether.

As with many biological phenomena—height, for example—propensity for promiscuity in either sex might be expected to be normally distributed; that is, to follow what are known colloquially as “bell curves”. The peaks of these curves would have different values between the sexes, just as they do in the case of height. But the curves’ shapes would be similar.



Rafael Wlodarski of Oxford University wondered whether things are a little more complicated than that. Perhaps, he and his colleagues posit in a study just published in Biology Letters, rather than cads, dads and their female equivalents simply being at the extremes of a continuous distribution, individual people are specialised for these roles. If so, the curve for each sex would look less like the cross-section of a bell, and more like that of a Bactrian camel, with two humps instead of one.

The mating game

To test this idea the team looked at two sets of data which had been collected for other purposes. One was from almost 600 people who had completed the “sociosexual orientation inventory”, a questionnaire intended to elucidate the different tendencies of people to engage in sexual relationships without a deep emotional commitment. The other was of 1,300 people who had had the lengths of their index and ring fingers measured. The ratio of these lengths indicates the effect on an individual of exposure to testosterone in the womb. (A long ring finger compared to the index finger means a big effect.) This ratio corresponds, throughout the primates as a group, to the amount of promiscuity found in a species’s mating system.



Both of these tests confirmed the idea that men are more predisposed to promiscuity than women. But Dr Wlodarski knew that already. What he wanted to determine was whether distinct sexual strategies exist in either or both sexes. Doing that from the relatively small samples available meant putting them through two statistical tests that asked how likely it was they really did come from a bimodal, camel-shaped distribution rather than bell-shaped one.



For both sexes, in the case of the sociosexual results, they clearly were bimodal. That pattern remained when the sample’s American and British participants were analysed separately. In the case of the finger data, all of which came from British participants, the men, but not the women, were bimodal.



These results suggest that—probably for men and possibly for women—caddishness, daddishness and so on are indeed discrete behavioural strategies, perhaps underpinned by genetic differences, rather than being extremes of a continuum in the way that tall and short people are. Although there is some overlap between the two strategies, they are, if Dr Wlodarski and his colleagues are correct, what biologists call phenotypes. These are outward manifestations of underlying genes that give natural selection something to get hold of and adapt down the generations.



Intriguingly, the difference in phenotypic mix between the sexes is not huge. Dr Wlodarski and his colleagues calculate that cads outnumber dads by a ratio of 57:43. Loose women, by contrast, are outnumbered by their more constant sisters, but by only 53:47. Each of these ratios tends in the direction of received wisdom. Both, though, are close enough to 50:50 for that fact to need an explanation.



In the case of men, there is a likely explanation. The reproductive output of a male of any species (measured as the number of his offspring that survive to adulthood) is limited mainly by the number of mates he can inseminate. He therefore has an incentive to be promiscuous (which will promote caddishness). But humans are unusual in that a father often helps care for his offspring. Those offspring are (at least, in a state of nature) less likely to survive and thrive without him. That will promote daddishness.



A woman’s evolutionary reasons to play fast and loose are less obvious, for having many lovers will not bring her more children. It will, however, bring her children who are more genetically diverse—and that might be an advantage in itself. Such children would, for instance, be less susceptible to catching the same diseases as each other, since openness to any given infection is partly determined by genetics.



If their analysis is correct, Dr Wlodarski and his colleagues have probably stumbled on a type of equilibrium known to biologists as an evolutionarily stable strategy, in which a way of behaving becomes more advantageous as it gets rarer, and less so as it gets commoner. Cads succeed when dads are frequent, and vice versa. Neither can conquer and neither can vanish. Such equilibria are part of a branch of maths called game theory—a name both men and women might think eminently appropriate.