One of the notable changes in higher education these days has been a steady increase in the proportion of women, not only in specialties that had previously been male bastions (medicine, veterinary science, law, science in general), but in college and university enrollments as a whole. It is hard not to be aware of the sex ratios around us. In a recent post, for example, Michael Ruse noted that his son, deciding which university to attend, is quite aware that one unit of the University of Toronto currently enrolls four women for every man.
Less well known, I suspect, is that the sex ratio of natural populations has been the subject of intense research among biologists, yielding what has emerged as one of evolutionary biology’s more robust formulations. Following is a quick-and-dirty summary of the evolution of sex ratios, after which we’ll return briefly to the question of sex ratios in that “unnatural” population, the college campus.
In the great majority of sexually reproducing animal populations, the sex ratio is remarkably close to 50:50. Biologist/mathematician R.A. Fisher intuited that this was likely due to the fact that among standard diploid species, the fitness of males and females is precisely equal (since every offspring represents, equally, the success of a mother and a father). Fisher showed that as a result, any deviation from equal sex ratios would necessarily be self-correcting, via natural selection.
Imagine a population in which for whatever reason the sex ratio had deviated so that there is currently an excess of males. Under this condition, parents who produce sons will be less fit—i.e., their reproductive success will be lower—than parents who bias their reproduction toward daughters. This is because whenever there is an excess of either sex above 50:50, individuals of the more abundant sex will on average be less successful in finding a mate than will individuals of the rarer sex. The result is “frequency-dependent selection,” in which any deviation, any temporary success, necessarily leads to selection against the more abundant form, and thus, a sex ratio that tends strongly toward equilibrium.
It is intriguing, and somewhat counter-intuitive, that this principle holds even among strongly non-monogamous species. Thus, imagine a harem-keeping species in which one male mates with 10 females, thereby leaving nine males as unfulfilled bachelors. Even here, it pays parents to produce equal numbers of males and females rather than biasing their reproduction to favor either sex! Lets say that in this species, females produce one offspring per year. The fitness of an average female is therefore one. But it fact, the fitness of the average male is also one, because there would be nine males with a fitness of zero for every one male with a fitness of ten. In this case, producing males is a riskier proposition, since nine males will fail for every one who succeeds … but the success of this 10th male will precisely make up for the failures of the preceding nine.
As a result, even when the mating ratio is strongly asymmetric, the average fitness payoff for parents is precisely the same, one, regardless of whether it is achieved by putting one’s bets on sons or daughters.
Back on campus, I find myself wondering whether something analogous won’t happen with regard to enrollment sex ratios. I could imagine, for example, that Michael’s son might figure that if he attends a school with an “excess” of young women, his situation as a young man would give him a romantic edge … and a similar process would likely operate for women contemplating enrolling in a school in which, say, women were under-represented.
Of course, the evolutionary biology of sex ratio is not directly importable onto the psychology, sociology, and economics of college choice. Biologists make the simplifying but reasonable assumption that mating and reproductive prospects are the primary drivers of animal behavior and ecology, including sex ratio selection. I’d like to think that prospective college students take other things into consideration, at least sometimes. But I’m not so sure.
