Infidelity

While society has always accepted that men engage in (or want to engage in) short-term sexual liaisons, women have sometimes been seen as 'pure' and entirely faithful. Evolutionary sexual psychologists, on the contrary, argue (pdf) that men and women pursue mixed sexual strategies, that is, are prone to engage in both long-term and short-term mating. Roughly speaking, the optimal strategy for a woman is to have a long-term relationship with the best mate she can attract but then, subject to an assessment of the risks, to have short-term sexual relationships with higher quality men (pdf). (Since the average woman is more likely to attract the highest quality men into a short-term rather than long-term relationship, she can often increase her fitness by engaging in extrapair copulations. If she can convince her primary partner to care for the children that (sometimes) result, so much the better). The optimal male strategy, on the other hand, is quite a bit more promiscuous: attract the highest quality mate possible to a long-term relationship (at least usually) and then, again taking account of risks, engage in as many extrapair copulations as possible, whatever the quality of the female involved. (As should be clear, I'm ignoring homosexual relationships for present purposes).

All of the above, however, is theory-driven and it leaves open the question of how prevalent infidelity is in the modern world. Determining the true incidence of infidelity is extremely challenging - not least because people find it difficult to admit to it - but we do have some idea. There is fascinating research, for example, showing that a not insubstantial proportion of children - at least 10% - are fathered in extrapair copulations. And a recent review of the literature found that infidelity occurs in about 25% of long-term heterosexual relationships in the United States. The latest is that the New York Times published a pretty good article summarizing some of the recent research and some recent trends, again in the United States. The most titillating finding is that there seems to be a trend towards greater female infidelity, but whether this is due to more honesty on questionnaires or more actual cheating is difficult to determine. What is clear, though, is that sexual fidelity is hardly universal and women are not necessarily less fallible in this regard than men. Obvious, perhaps, but many have denied it...