Each hair sits in a cavity in the skin called a follicle. Baldness in men occurs when the follicle shrinks over time, resulting in shorter and finer hair. The end result is a very small follicle with no hair inside. Ordinarily, hair should grow back. However, in men who are balding, the follicle fails to grow a new hair. Why this occurs is not well understood, but it is related to your genes and male sex hormones. Even though the follicles are small, they remain alive, suggesting the possibility of new growth.
Classic male pattern baldness is usually diagnosed based on the appearance and pattern of the hair loss. Any atypical hair loss may be caused by other medical disorders. A skin biopsy or other procedures may be needed to diagnose other disorders that cause loss of hair. By age 35, two-thirds of American men will have some degree of appreciable hair loss and by age 50 approximately 85% of men have significantly thinning hair. About 25% of men who suffer from male pattern baldness begin the painful process before they reach 21.