Where the scale comes from
In 1951 the American researcher James Hamilton described the patterns in which men lose scalp hair. In 1975 O'Tar Norwood revised those drawings after examining 1,000 adult white men, when the growth of hair transplant surgery called for a shared, reproducible language. That is why the scale is usually called Hamilton-Norwood. It was built for male-pattern hair loss, the common inherited kind, and it is still the scale most clinics and studies use to say how far that pattern has gone.
Based on: Hamilton 1951; Norwood 1975
What the stages follow
The scale watches two areas at once. At the front, the hairline recedes from the temples, leaving the familiar M, and later moves back in the middle too. On top, the crown (the vertex) can thin into a round bald area. In the middle stages both areas grow while a band of hair still separates them; in the late stages the band disappears and the bare areas join. Norwood also described A variants, in which the whole front edge moves back without a separate bald crown. They are not rare: in one community study, 12 percent of men showed a mainly frontal pattern.
Based on: Norwood 1975; Gupta & Mysore 2016; Rhodes et al. 1998
Norwood 2 vs 3: where the line falls
Stage II means triangular, usually symmetrical recession at the temples, while the middle of the hairline moves back much less. Stage III means deep recession at the temples, with the corners bare or only thinly covered, and Norwood treated it as the least hair loss that counts as baldness. In practice the step between them is a matter of degree, not a sharp line. Three things help: how far each corner sits behind the middle of the hairline, whether the hairs in the corners are finer and shorter than those behind them, and whether the corners keep moving from one season to the next. A head on the boundary is best described as II to III, and the trend over months tells you more than the label.
Based on: Norwood 1975; Gupta & Mysore 2016
How common each stage is
Pattern hair loss is common and usually slow. In a community sample of healthy men aged 18 to 49, 42 percent were at stage III or beyond: 16 percent of men aged 18 to 29 and 53 percent of men aged 40 to 49. The American Academy of Dermatology notes that it can begin in the late teens or early twenties, though it usually appears later, and that by 50 more than half of white men show a visible sign, such as noticeable thinning, a receding hairline or balding. The pace varies widely, so a stage describes where a pattern is today, not how fast it will move.
Based on: Rhodes et al. 1998; American Academy of Dermatology
Why experts disagree, and why a range is honest
In one study, seven dermatologists and sixteen dermatology residents classified the same 43 photographs, and the authors judged their agreement unsatisfactory; in a second round, raters who scored the same pictures again three months later often did not repeat their first scores. Agreement is better in person: two trained observers who examined the same men agreed exactly 85 percent of the time, while the men's own ratings matched the observers only about half the time. Light, camera angle, hair length and styling all move the picture. Newer systems such as the BASP classification describe the hairline shape and the density on top separately, for men and women alike, but none removes the need for judgement.
Based on: Guarrera et al. 2009; Taylor et al. 2004; Lee et al. 2007
Reading your own photos against the scale
Take one photo from the front with your hair dry, unstyled and pushed back, and one from above the crown, both in bright, even light without flash. Look at the temples and the crown separately, then pick the two neighbouring drawings that fit best. Compare with your own earlier photos more than with the drawings: the same setup every one to three months shows change far better than a single picture. The scale cannot tell why hair is thinning. Sudden shedding, round bald patches, a sore or flaky scalp, or loss of eyebrows or body hair are reasons to see a dermatologist rather than to count stages.
Based on: American Academy of Dermatology