Boys who attend high school with too few girls may suffer the consequences long after prom. Men who reach sexual maturity where they outnumber women live three months less than the average life expectancy among peers whose competition for a mate isn’t as fierce, an HMS-led study suggests. The steeper the gender ratio, the shorter the lifespan.
Gender ratios and longevity have been linked in animals. To learn whether a connection exists in humans as well, Nicholas Christakis ’88, senior author on the study and an HMS professor of medicine and of medical sociology, collaborated with researchers in the United States and China. Looking at men who graduated from Wisconsin high schools in 1957 and, separately, at a national sample of more than 7 million U.S. men, the researchers saw similar results. Their findings appeared in the August issue of Demography.
The social costs of gender imbalances are well documented in China and India, where selective abortion and other factors have led to men outnumbering women by 20 percent in some regions. Such male-dominant environments, already linked to greater violence and human trafficking, may shorten life as well.
Christakis suspects that a combination of social and biological factors may account for this curtailment. Finding a mate can add stress—a known health hazard. “We literally come to embody the social world around us,” Christakis says, “and what could be more social than the dynamics of sexual competition?”