Fun with sex

One important factor that drove the evolution of psychological sexual dimorphism is the difference in the minimum obligatory parental investment between the sexes. (This is the great insight of Trivers, 1972). Men, as the rather coarse saying has it, ‘can leave a bed unmade’. That is, a man need only invest a few minutes of effort and some sperm to produce a child. Women, on the contrary, must invest as much as men plus 9 months of pregnancy and, given the absence of baby formula on the African savannah, several months or years of breast-feeding. Moreover, before the advent of modern medicine, childbirth was very dangerous so a woman quite literally risked her life to have children. The minimum obligatory investment for men and women, then, is radically different, so we should expect the evolution of a dimorphic sexual psychology reflecting, as Trivers put it, 'female choice and male competition'. (This is, obviously, a crude simplification). And, not surprisingly, we have a mountain of empirical evidence that confirms this expectation.

There is a lot one can say about this theory, and the above sketch certainly does not do it justice or acknowledge the complexities and uncertainties of the empirical data. But a story I saw in a newspaper recently made me think of one of its features, namely, that a man could always be doing better. From the perspective of a man’s genes, women are an extremely valuable and limiting resource. This may seem a bit weird, so let me explain. There are (of course) a finite number of fertile women alive at any given time, and, since a man has such a low minimum parental investment, he could, in principle, impregnate tens of thousands of them. Women, on the other hand, have to carry and give birth to all their offspring, so the total number of children each woman could have in a lifetime is severely limited by comparison. Men have the potential to sire several orders of magnitude more offspring than women, and as a result there is an oversupply of willing males. (One interesting consequence is that there is a much greater variance in male reproductive success, which produces much greater variance in males in a whole range of traits. The variance in male mathematics grades, for example, is substantially higher than that of women).

In any case, the story that got me thinking about this again concerns one Desmond Hatchett (pictured above). Hatchett, an American man from Tennessee, is only 29 years old but, amazingly, has fathered 20 children. Not quite Ismial the Bloodthirsty (who reportedly sired at least 888 children) or Genghis Khan (who is the likely ancestor [pdf] of ~8% of Central Asian men, and ~0.5% of all men worldwide), but evolutionary speaking, not bad at all.

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Trivers, R. (1972) "Parental investment and sexual selection" in Campbell, B. (ed.), Sexual Selection and the Descent of Man.

Video: Ten things you didn't know about orgasm (NSFW)

Mary Roach, science writer and author of Bonk: The Curios Coupling of Science and Sex, gave an excellent and funny talk at TED earlier this year about the science of sex. (Embedded below, or click here). Warning, this is mildly NSFW.

Oh. Hold out for the pig farmers bit, it's priceless...

DNA & Dating

I blogged back in January about ScientificMatch, a dating service that, perhaps rather unromantically, matches people based on their genes. I see New Scientist magazine has caught on, and has published a great article about this rather odd development. The author of the piece, Linda Geddes, summarizes the scientific rationale for the service, delves into several of the criticisms thereof and, most interestingly, subjects herself and her fiancée to the testing. Good stuff indeed.

My bottom line, for what it's worth, is that it's plausible to think this kind of genetic matching is an improvement over sheer chance, but I doubt very much it'll be superior to our evolved sexual psychology. (Although I do suspect it'll in general predict compatibility better than questionnaire-based online matching services). So, if you're wealthy and lonely, give it a try... otherwise, stick to the singles bars.

The dizzying diversity of human sexual strategies

New Scientist magazine has an interesting article on the huge diversity of human sexual strategies. The article mainly focuses on a construct known as sociosexuality, the tendency to prefer either restricted or unrestricted sex. An excerpt:

Of course, it is not that simple. Women can be as sexually unrestrained as men. In fact, there is a huge overlap in the sociosexuality scores of men and women, with more variation within the sexes than between them. Some researchers are now trying to explain these subtleties in terms of biology and evolution.

Take the fact that women's interest in casual sex can vary wildly over time. A hint that these short-term sexual encounters might have biological and evolutionary advantages comes from the timing of them. Several studies have shown that women are more likely to fancy a fling around the time they are ovulating - although there is no suggestion that this is a conscious decision. Not only that, says David Schmitt of Bradley University, Illinois, women show a shift in preference to men who look more masculine and symmetrical - both indicators of good genes. Women may have a dual strategy going, suggests Schmitt. "Humans infants need a lot of help, so we have pair-bonding where males and females help raise a child, but the woman can obtain good genes - perhaps better genes than from the husband - through short-term mating right before ovulation."

This rather reminds me of David Buss's answer to this year's Edge.org question, "What have you changed your mind about?" (which I blogged about here). Buss said he had realized that female sexual psychology is significantly more complicated than he had previously thought. Just so.

More Infidelity

Talking about infidelity... New Scientist has a report on a recent study in Human Nature that concluded men are better at detecting infidelity than women, but that women are adept at hiding it. A paragraph or two:

"This adds to the evidence that men have evolved defences to detect their partner's infidelity," says David Buss at the University of Texas, Austin. He adds that it demonstrates a "fascinating cognitive bias that leads men to err on the side of caution by overestimating a partner's infidelity".

Andrews suggests that women have countered this by becoming better at covering up affairs. Complex statistical analysis of the data hinted that a further 10 per cent of the women in the study had cheated on top of the 18.5 per cent who admitted to it in the questionnaires, whereas the men had been honest about their philandering.

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...