A reader writes:
I'm a graduate student taking an evolution seminar right now. We were discussing phenotypic plasticity today and read an article from 1970 called, "The Effects of Sexual Activity on Beard Growth in Man" (PDF). I thought you and some of your readers would get a kick out of it. It was published in Nature, one of the most respected, if not the most respected, science journals out there. Money quote:
Perhaps the most interesting feature of these changes was the way the increase in beard growth anticipated the resumption of sexual relationships. The longer the period of abstinence, the more obvious was the anticipatory response (beard growth).
There are some great figures too.
Testosterone is the key of course.