Why do humans have such giant willies when the rest of the primates have tiny willies? #explaintomelikeI’mfive #eli5

Question

Why do humans have such giant willies when the rest of the primates have tiny willies? #explaintomelikeI’mfive #eli5

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Answer 1

One hypothesis is the female vagina has to be larger to birth the larger heads of human babies (thus the penis has to be larger to be stimulated to ejaculation by the larger vagina)

Another one is the promiscuity theory. In this one, it is thought that females are less selective in the lower rungs of primate groups and many males get to mate w many females. Humans tend to be more selective and that through most of pre civilization humanity females were much more selective.