http://www.biology.mcmaster.ca/course_info/2l03/Nature-Beards1970.pdf
This is a self-study published in Nature by an anonymous author in 1970. The reason for anonymity is quite apparent. He lived in isolation for most of the week, and would return to the mainland on weekends. For 2 years, he would shave each morning and weigh the amount of beard shaved off. He found that his beard growth peaked on Fridays, which he correlated with anticipation of going back to civilisation and thus having the chance to have sex. The increase in beard growth is a product of an increase in androgens in the body, which, as most people probably know, can be caused by thinking about sex.
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