Sorry bald men, but it’s not true. It is often assumed that because the male hormone testosterone causes male pattern hair loss, a balding man must have high levels of it and so must be more virile than a man with a full head of hair. However, it is not so much the level of hormone which causes this type of hair loss, but how the hair cells respond to it - and this is genetically determined.
The science bit:
It is not, strictly speaking, the male hormone testosterone that causes male hair loss. For this type of hair loss, testosterone must be converted by an enzyme called 5-alpha reductase (located within the forming hair cells) into a much more potent hormone, dihydrotestosterone (DHT). It is the effect of this hormone that inhibits the growth of new hair cells, resulting in thinner hair (or no hair) being produced. It means that a man can have a very high level of testosterone but if he has not inherited the ability for testosterone to convert into DHT, then no significant hair loss will occur.