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I just want to report the finding of one paper here. I don't know much about the rest of the literature and can't comment on it. Otto et al. (2015) investigate this question in sexually reproducing organisms. As I understand it, they assume that the amount of time spent in on phase of the life cycle is proportional to the amount of selection happening in ...


What gender someone identifies with or relates to comes in to play outside of utero as kids start to perceive themselves in a certain way. Biological factors, however, influence the likelihood someone will perceive themselves in a certain way. These factors may appear in utero or even later in life. Studies have suggested that developmental factors that ...


This is a hypothesis that I found in Desmond Morris book, The naked ape. Basically, humans transit from a rear-penetration position to a frontal-penetration position. In this transition, the sexual clue performs by buttocks was perform now by breast. In short, according to Morris: breasts are sexual clues because they look like buttocks.


Short answer, we don't know. There are several competing hypotheses but they are all nearly impossible to test. Behavior often suffers the same problem as testing, we can come up with a hypothetical reason but since we are the only organism that does it, we have no good testing options. Secondary sexual characteristics in general have problems with testing....

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