Hereditary-pattern baldness is the most common cause of hair loss. Hereditary-pattern baldness is not really a disease, but a natural condition caused by some combination of genetics, hormone levels and the aging process.
Almost all men and women will notice hair loss or hair thinning as they age. However, up to 40% of men and women will experience a more obvious form of this condition. Hair loss typically begins in the 20s and 30s, although in women the changes are most noticeable after menopause. The condition is also called androgenetic alopecia and, in men, male-pattern baldness.
Researchers have begun to understand more about the cause of this type of hair loss. Under the influence of a form of the male hormone testosterone, the normal cycle of hair growth changes, resulting in shorter, thinner or "miniaturized" hair. Eventually, hair growth in certain parts of the scalp stops entirely, which causes the typical pattern of hair loss. Contrary to the folk wisdom that baldness is inherited from one's mother's family, the condition seems to depend on genes contributed by both parents.