The most common type of baldness is called male pattern baldness or more scientifically, androgenetic alopecia.

In androgenetic alopecia, hair follicles that are producing healthy (terminal) hairs begin to produce thinner, shorter, more brittle hairs with weaker shafts. Eventually, these follicles produce only fine, almost invisible, short (vellus) hairs, or they may die out altogether.

Androgenetic hair loss is caused by the effects of male hormones on genetically susceptible hair follicles. It is related to three interdependent factors:

Genes
Common baldness cannot occur without the presence of specific inherited genes. These genes can be passed on by either parent. The genetics of androgenetic alopecia is complicated, and hair loss is now felt to involve more than one gene. It is felt that the genes governing common baldness are autosomal. This means that the baldness trait can be inherited from the mother's side of the family or the father's side. The commonly held notion that baldness comes only from the mother's side of the family is incorrect, although for reasons not fully understood, the predisposition inherited from an affected mother is of slightly greater importance than that inherited from an affected father. The genes involved in androgenetic alopecia are felt to be dominant, meaning only one gene of a pair is needed for the trait to show up in the individual.

The ability of a gene to affect one's characteristics is called expressivity. Gene expression is related to a number of factors, the major ones being hormones and age, although stress and other factors can play a role.

Hormones
The major male sex hormone is called testosterone. The hormone felt to be directly involved in androgenetic alopecia is actually dihydrotestosterone (DHT), a much more biologically active androgen than testosterone. DHT is formed by the action of the enzyme 5 alpha reductase on testosterone. DHT acts by binding to special receptor sites on the cells of the hair follicles to cause the specific changes associated with balding. Among other effects, DHT decreases the length of the growing (anagen) cycle, and increases the resting (telogen) phase and with each new cycle the hair shaft becomes progressively smaller (miniaturized).

In men, 5 alpha reductase activity is higher in the balding area. This is the enzyme that is inhibited by the hair loss medication finasteride (Propecia). Women have one half the amount of 5 alpha reductase overall as compared to men, but have higher levels of the enzyme aromatase, especially in their frontal hairline. Aromatase decreases the formation of the DHT, and its presence in women may help to explain why the presentation of hair loss in women is different than in males.

The tendency to develop hair loss requires time of exposure of the susceptible hair follicles to the hormone for hair loss to begin. The time required for this to start varies from one individual to another and is related to a person's genetic expression and to the levels of androgen hormones in their bloodstream. There is another time factor that is poorly understood. Hair loss does not occur all at once, nor in a steady, straight-line progression. Hair loss is cyclical. People who are losing their hair experience alternating periods of slow and rapid hair loss and even stability. Many of the factors that cause the rate of loss to speed up or slow down are unknown.

Age
The age at which hair loss finally manifests itself varies from one individual to another and is related to a person's genetic composition and to the levels of androgen type hormones circulating in their bloodstream. Although many of the factors that cause the rate of hair loss to speed up or slow down are unknown, we do know that with age, a person's total hair volume will decrease. In all adults, the entire scalp undergoes this aging process. Even the donor zone is not truly permanent, but will gradually thin to some degree over time. Fortunately, in most people, the donor zone retains enough permanent hair that hair transplantation is a viable procedure even for patients well into their 70s.