If you’ve ever been to a junior high school (aka middle school), either as student or adult observer, you’ve noticed that the girls tend to be substantially taller and more mature—socially as well as sexually—than the boys. You may even have wondered why. Here goes.
The phenomenon in question is known as “sexual bimaturism,” and it occurs in a wide range of animal species, nearly always for the same reason: because the sex engaging in higher levels of social and sexual competition becomes sexually mature later, which in turn is due to the fact that it can be bad for your health (and thus, your fitness) to enter the competitive arena when too small, weak, young and/or inexperienced. In most cases, that sex is male.
Such an outcome is as counter-intuitive as it is widespread. Consider that among mammals, for example, it is the females who carry out pregnancy, give birth, and then nurse their young (lactation, as a matter of fact, requires significantly more physiological investment than does nourishing offspring in utero). Males, by contrast, need only contribute a squirt of sperm. There is simply no comparing the former’s metabolic expense to the latter’s. One might expect, therefore, that nearly all male mammals would begin breeding when quite young whereas females would hold off until they are older, larger, stronger, and altogether more able to withstand the greater demands unavoidably imposed upon them when it comes to reproduction. But as noted, the actual pattern is precisely reversed.
Evidence as to why comes from comparing closely related species that differ in their “degree of polygyny,” the extent to which the animals in question reproduce via a “harem” social system (technically, the breeding ratio, or more technically and accurately yet, the ratio of male to female variance in reproductive success). Considering members of the seal and sea lion family, for example, some are essentially monogamous (e.g. harp seals), whereas others, such as elephant seals, may form harems in which up to 40 females are mated by a single harem-keeper, thereby leaving 39 frustrated, unsuccessful bachelor bulls. And others are in-between. Here is the key finding: Among the more monogamous species, males and females become sexually mature at about the same time, and male-female size differences are very slight. The greater the degree of polygyny, the greater the male-female difference in size (“sexual dimorphism”) and in sexual bimaturism. In fact, knowing only the sexual bimaturism or sexual dimorphism, one can predict the extent of polygyny, and vice versa.
Significantly, the same pattern holds for numerous other species: cervids (members of the deer family), non-human primates, gallinaceous birds (grouse and wildfowl), etc. In all these cases, the greater the disparity in variance of reproductive success-essentially the larger the average harem size-the greater the age and size difference when it comes to male-female age and size at sexual maturity. And it is always the harem-keeping sex that matures more slowly and that ends up being larger, as well as more aggressive and violence-prone.
Additional evidence: Among those very rare species that are polyandrous—forming “reverse harems” in which one female mates with numerous males and among which females are therefore the more competitive sex—the usual pattern of sexual bimaturism is reversed: Females begin breeding later than males, waiting until they are older, larger, stronger, and therefore more likely to be competitively successful.
Back to Homo sapiens. A Martian zoologist visiting planet Earth would have no doubt that human beings are biologically inclined toward polygyny: Not only was this the preferred social system for more than 80 percent of human societies prior to the cultural homogenization that came with Western colonialism, and not only are men on average larger and more aggressive than women, but our pattern of sexual bimaturism fits right in.
Note: Such consistent patterns (complete with expected reversals when the breeding system is reversed) do not prove the above contention, but they provide very strong evidence. Moreover, this apparent fact doesn’t say anything about human polygyny being “good,” just because it is natural. After all, typhoid is natural, as are tsunamis. Nor does it indicate that monogamy and sexual equality are impossible, or undesirable. But it certainly goes a long way toward explaining why the girl I fancied during ninth grade was a good two inches taller than me, and why she was much more interested in another guy … comparable in physical and social maturity to herself, and who was also three years older.