TESTOSTERONE gradually declines with age, right? Not for the Ariaal – subsistence pastoralists living in northern Kenya. They experience a decline in levels of the male hormone only when they get married. The finding provides a social and evolutionary explanation for the decrease in testosterone, rather than an age-related one.

Ariaal men remain single “warriors” until they are around 30, at which time they marry one or more women. Peter Gray of the University of Nevada, Las Vegas, and colleagues measured testosterone in 205 Ariaal men and found that those with one wife had lower levels of the hormone than unmarried men, and men with more than one wife had the lowest levels …