@hairsuit , Here is my take....Testosterone and DHT grow muscle and bone. So in some men with tight scalps, test or DHT causes muscle growth or even DHT causing skull growth. Men with stress and have round heads, causes major scalp tension on the top. I think this explains why it is only the top that loses hair. Also, this explains why its in a horseshoe pattern. the hair loss is even and symmetrical over the top. because the muscles on the sides are pulling downward on all sides of the scalp.
Women are mostly protected because of progesterone and estrogen. Women have much less muscle mass. Due to anti-testosterone , Progesterone.
Look and female steroid bodybuilders. their heads and foreheads start to get like a man's, once they get huge muscles like a man.
Sex and masturbation cause histamine and eventually Prolactin. Prolactin constricts the arteries, and the scalp. this is what POIS is. High prolactin without enough Oxytocin, keeps the arteries constricted, not allowing testosterone and DHEA to heal after an orgasm. testosterone is needed for hair growth. but in some men, head shape, stress, and tight scalps make it impossible to grow hair , since blood flow reduced. but the sides of the head grow normal, and so does the beard.
It is presumed that finasteride works by reducing DHT, thus reducing muscles around the perimeter or sides of the head. Most men who complain about sides on fin, have lowers strength and cracking joints. I think they even might reverse skull expansion to some extent. thus allowing a less tight scalp.