To see why this is surprising, consider that 1 male can impregnate 2, 10, or 100 females; it wouldn’t seem that you need the same number of males as females to ensure the survival of the species. This is even more surprising in the vast majority of animal species where the male contributes very little to raising the children—humans are extraordinary, even among primates, for their level of paternal investment. Balanced gender ratios are found even in species where the male impregnates the female and vanishes into the mist.
