There are many causes of baldness, including serious illness, reaction to certain medication and stress, but by far the most common is Male Pattern Baldness (MPB) or Andogenic Alopecia. MPB is caused by a combination of two factors: a genetic predisposition and Dihydrotestosterone (DHT).
DHT is a byproduct of testosterone, and is therefore present in every male. Scientists have shown that DHT shrinks hair follicles, and that if DHT is suppressed, the follicles will continue to thrive.
In 1949 a landmark study was performed that explained the causes of MPB. A group of castrated convicts, males who were therefore not producing testosterone, were divided into two groups. The first group were those who had a family history of MPB, and the second group were those with no family history of balding. Both groups were injected with testosterone; the group with a family history of MPB began losing their hair within weeks, while the group with no family history lost no hair, but did begin to develop a mature hairline. When the injections were stopped, the progression of thinning stopped.
Looking past the inhumane and surprising extremes of this experiment, it was discovered that MPB was an inherited trait that is triggered with the presence of testosterone. Since that experiment, we have learned that it is not testosterone but the testosterone byproduct DHT that causes hair loss.
Hair follicles that are sensitive to DHT must be exposed to the hormone for a prolonged period of time for the affected follicle to complete the miniaturization process. Today, with proper intervention, this process can be slowed or even stopped if caught early enough.
What MPB sufferers inherit are hair follicles from their mothers or fathers with a genetic sensitivity to DHT. Hair follicles sensitive to DHT begin to miniaturize, shortening the lifespan of each hair follicle affected. Eventually, these follicles stop producing cosmetically acceptable hair.
MPB is characterized by a receding hairline and thinning in the temple, crown or vertex area. These areas are the most sensitive to DHT. For some men, DHT even thins the hair on the back of the head, although this area is usually spared, leaving a permanent donor area that can be used for modern hair transplants.
There are still many factors about the inheritance of baldness that we don't understand. I often see a male with a strong family history of MPB that says he has an older brother with little or no balding. As well, some young adult males bald rapidly up until the age of 35, whereupon the balding pattern stops. In contrast, males can also experience little or no thinning up until their early 40s, and then lose all or most of their hair in the next 20 years, balding almost completely by age 60. Unfortunately, we cannot predict the exact course each individual's balding will take when he is in the early balding stages. But generally speaking, the earlier you start losing hair, the more hair you are going to lose.