For those of you that don't know, the human penis is disproportionately larger than that of other apes. It's being hypothesized that this is caused by the increase in human cranial size. Bigger brains obviously lead to larger craniums, however this meant that the human vagina also had to be bigger. In order to have satisying sex, the human penis also became bigger. Males with larger penises faired better through natural selection as women preferred them to males with smaller penises as partners for sex. The article concludes that in general females prefer males with larger penises, and probably have since prehistoric times. [1]

[1] https://commerce.metapress.com/conte...ringerlink.com