This is the most common cause of hair loss. The development of genetic hair loss is associated with the shortening of the anagen (growing) phase of the hair cycle and consequently with an increase in the proportion of telogen (resting) hairs. There is a reduction in the size of the affected follicles, which results in a reduction in the diameter of the hairs that they produce. This is an essential feature of this type of hair loss, which accounts for the thinning of the hair and the widening of the partings.
The onset of male genetic hair loss is linearly related to age; that is, 20% of men experience some hair loss by age twenty; 30% of men experience some hair loss by age thirty, and so on. The hair begins to recede at the temples and thin in the vertex (crown) area. Eventually, the entire fronto-vertex (between the hairline and crown) area of the scalp can be involved.
In order to assess the extent of genetic hair loss in men, dermatologists and trichologists commonly use the Hamilton-Norwood scale. The Hamilton-Norwood scale helps hair loss experts and patients alike monitor the state of loss, devise an appropriate treatment regimen and evaluate whether the condition is improving or worsening.
Source: Hamilton JB. Patterned loss of hair in man: types and incidence. Annals of the N.Y. Academy of Science, 1954; 53: 708-728.