Human sexual size dimorphism, the difference in height between males and females, is often touted as a classic example of sexual selection: the kind of trait whose evolution was driven by differences in reproductive success rather than survival. But [researcher Holly] Dunsworth thinks theorists leaped to that conclusion too easily.
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Dunsworth found a more direct explanation when she dug into the literature on bone biology and development, focusing in particular on how those relate to hormones. That’s when it became clear to her that “women are shorter than men because most of them have ovaries.”
Ovaries matter because they produce a lot more estrogen than testes do, and estrogen helps direct bone development. “In all human skeletons, a lot of estrogen stimulates long bone growth,” Dunsworth explained. Before puberty, people with ovaries and people with testes grow at roughly the same rate. Then those with ovaries ramp up estrogen production, which stimulates the growth plates in their bones and causes the long bones in particular to lengthen. That’s why, during early adolescence, girls are generally taller than boys.
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“We often have this notion that male bodies are the default and women’s bodies are a deviation from that default. If we don’t start with one body as the default, we look at how these processes lead to sex differences, then you can start testing hypotheses much better,” [Dunsworth said.]
