He had a unique explanation for physiological changes in men and women
The age at which a girl first starts her period can reveal a great deal about both her physical health and her social context.
In Western countries, the average age at menarche fell from 16.5 years in 1840 to 13 in the 1990s. Most medical historians believe this to have been the result of improved nutrition and reduced impact of infectious disease in childhood. The richer societies became, the earlier girls began puberty.
But, peculiarly, since the turn of the century that relationship between affluence and age at menarche has gone into reverse. Now it is girls of lower socioeconomic status who get their periods first.
