Among humans, twelve million sperm per hour in men vs. four hundred eggs over the course of a lifetime in women. Higher gamete production is, indeed, the biological definition of “male”; when biologists discover an exotic new species, they determine which sex is which by comparing their rate of gamete production.

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Even slight differences between genetic fitness in men can be perceived by women, who are naturally attuned to making such comparisons, and they translate into big differences in male reproductive success. This is especially true in a polygamous society, but the same effects occur in a more muted way under a system of monogamy, with males perceived as fit tending to marry earlier with – on average – more fertile females.

Most of what follows derives directly or indirectly from these simple facts.