From Of Beards and Men, by Christopher Oldstone-Moore, which will be published next month by the University of Chicago Press. Oldstone-Moore is a senior lecturer in history at Wright State University, in Dayton, Ohio.

Philosophers and physicians of classical Greece, including Hippocrates, believed that hair in general, and the beard in particular, was a manifestation of male superiority. Men had greater “vital heat” than women or children, and this heat accounted for the greater hairiness of men, as well as their greater size, strength, and reasoning powers. The purified essence of this vital heat was semen. Introduced into a woman’s womb, it produced new life. Flowing within a man’s body, it produced a profusion of hair. Although women had some vital heat, no woman could sustain a masculine level. One medical text discussed the cases of two women who had stopped menstruating and started growing facial hair, after which they became sick and died. It was surmised that their bodies could not handle the abnormal buildup of semen caused by the failure of their feminine cycles. Only men’s bodies were strong enough to grow beards and remain alive.

The author of a medical text entitled Nature of the Child explained the mechanism thus: the flow of semen acted on certain kinds of porous flesh to cause hair to grow, much as a plant would grow from fertile soil. Because both men and women had semen, both sexes grew hair on their bodies, particularly on the head, where the semen was stored. But men’s bodies were different in two ways. First, they had greater heat, which made the skin more porous, and second, they had more semen. The beard could also be explained by the way semen flowed in the body. Stored in the head, it traveled to the rest of the body when necessary, particularly during sex. For men, the hair on the chin grew thick “when the fluid in its course from the head during intercourse is delayed by its arrival in the chin, which projects forward of the breast.” Presumably, men had sex facing downward most of the time. This made the face the repository of more semen and more hair growth.

Aristotle agreed that hot fluids were the ultimate source of hair, and that men with “strong sexual passions” had especially full, thick beards because of an abundance of semen. These men were also more likely to go bald from the depletion of semen through repeated intercourse. “That is why,” Aristotle concluded, “no one goes bald before the time of sexual intercourse, and also why that is the time when those who are naturally prone to intercourse go bald.”