Men are more likely than women to lose their hair because changes in testosterone levels are a major cause of hair loss. Women have less testosterone, so they tend to keep their hair, although it may thin a bit during menopause.
Natural hair loss, called androgen alopecia, occurs when an enzyme converts testosterone into dihydrotestosterone. Dihydrotestosterone causes hair to become thin and brittle, leading to permanent hair loss. Higher amounts of this hormone build up as men age, leading to progressive baldness. Some men appear to be genetically predisposed to hormonal changes earlier in life or more quickly. Researchers theorize that Asian men may have less dihdrotestosterone than Caucasian ones, because the former tend not to exhibit the same likelihood of baldness.
Women may notice hair thinning during menopause because their bodies begin to produce less estrogen, which increases the influence of testosterone on their hair. However, the lesser levels of testosterone lead to a more even thinning in most women rather than the typical male pattern baldness.
Some animals display similar patterns of baldness. Male chimpanzees sometimes lose the hair on their heads as they age. Stump-tailed macaques, a species of monkey found in southeast Asia, are unusual because both sexes lose the hair on their faces and heads.