Why is that males become bald?

Baldness or Androgenetic Alopecia (AP), is not restricted to males alone but occurs in females in a different way. Pattern baldness, has been a part of the human race for as long as we have historical records. Evolutionary evidence suggests baldness has been around longer than the modern human race. Our nearest non-human primate relatives, orangutans and gorillas can also develop the condition. It is a common form of hair loss and could be described as part of our general genetic phenotype. People who do not develop AP are in minority. We could even say these non-bald people are deviants from the norm! 

Despite its standard name of "male pattern baldness" AP is also the most common form of hair loss in women. 

It develops as a gradual reduction of scalp hair follicle size, and reduced time in the anagen active growth phase, leading to hair follicles in the telogen resting stage of the hair cycle.

In men, the hair loss is limited to the top of the head and can involve thinning and/or receding hair lines. In women the presentation is different with just diffuse thinning over the top of the head and sometimes thinning over the entire scalp.

In 400 BC Hippocrates observed that eunuchs did not become bald. Later it was concluded that there was a genetic cause behind the development of baldness. However, it is known it is known that it is more specifically a male hormone which is converted from enzyme testosterone which contributes to baldness in those who are genetically predisposed. When baldness occurs, large active hair follicles in specific areas begin to change to smaller less active ones that shrink slightly with each new growth cycle. Slowly hair shafts narrow producing progressively finer hairs with each new growth cycle until eventually the hair becomes transparent and stops emerging.

Medicine has long recognized baldness as an inherited systemic disease associated with sexual development. Ancient Greek doctors realized that male pattern baldness can develop in men of any age after puberty. They recorded that young boys castrated before puberty did not become bald regardless of their genetic family history. However, boys castrated during or after puberty developed conditions. We now know that the reason for their observation is castration pre-puberty stops hair follicles from being exposed to Androgens made by the gonads during adolescence. Castration after puberty is too late. Once hair follicles have been exposed to androgen they are fated to become androgen sensitive and cause baldness.        

Hair fall patterns in human male and female: