In a human body, there grows hair in every important area. Hair in each point has a role to protect the body, but especially hair of the head plays a role to control heat of the head.
It is considered that one who tends to have heat on the head has less hair, and on the other hand, one who should not cool one's head have a lot of hair. This is why there are less haired people in men, and more hair in women. The diffrent characteristics between sex are indicated here.
So, hair transplantation for men and frequent washing hair for women are unnatural in this point of view.
*This article is translated from "Yamai hitokuchi Memo" written by Shoji Kobayashi, 2005.