Growing Phase

In a normal scalp, about 90% of hair follicles are growing at any one time. Each follicle stays in the growing phase for two to six years. How long an individual hair grows depends on how long it is in the growing phase. After the growing phase, hair enters a brief transitional phase.

Transitional Phase

The transitional phase typically lasts two to three weeks. Less than 1% of your hair is in this phase at any one time.

Resting Phase

Hair follicles rest for two to three months. At the end of the resting phase, the follicle "sheds" its hair. Then the growing phase begins again.

In 1951 Dr James B. Hamilton developed the first useful classification system with which to guage the extent of hair loss due to pattern baldness in men. He examined 312 Caucasian males and 214 Caucasian females aged between 20 and 89 years to produce the scale. He used a classification system involving eight separate groups. In types I to III of the Hamilton classification system there is no obvious hair loss. Types IV to VIII categorized progressively more extensive hair loss.

Type I is reserved for people with no hair loss. Hamilton described it as "the abscence of bilateral recessions along the anterior border of the hair line in frontoparietal regions". In other words there is no evidence of a "widows peak". Hamilton also suggested a variation on type I classed as type IA. Type IA was reserved for men with a very high hair line on the forehead. However this high hairline was not due to any hair line recession. The high hairline was simply present as a result of genetic inheritance and family features.

In developing the scale, he recognised that up to 96% of men and 79% of women reach at least hair loss stage two after full pubertal sexual maturation. He claimed 58% of men over the age of 50 had hair loss resembling patterns type V to type VIII with the extent of hair loss stabilising at around 70 years of age. He also claimed that by 50 years of age 25% of women had developed type IV hair loss. The extent of hair loss apparently stabilised once women were over the age of 50.

Hamilton claimed that more than 99% of men could be classified according to his system. To prove the effectiveness and easy use of his classification system he
recruited 4 attendants at a mental institution with no scientific training. He gave each attendant a copy of his sketches and brief descriptions of the classifications. The attandants studied for half an hour and then used the information to classify 125 subjects previously examined by Hamilton. It was found that the attendants gave individuals the same classification as Hamilton 98% of the time. This was simple but persuasive evidence that the classification table was accurate and could be reliably used in a dermatology clinic.