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…

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