Humans are probably the only species on Earth who nurture their young for 20 years or more. For men in particular, the intensive demands of parenting can come as such a shock that a built-in biological mechanism has evolved to help cope with the change. A new study shows that becoming a father leads to a sharp decline in testosterone, suggesting that although high levels of the hormone may help men win a mate, testosterone-fueled traits such as aggression and competition are less useful when it comes to raising children.
Previous research had shown that among new fathers, testosterone levels were lower than in men of the same age who didn't have children. But no study addressed whether parenthood itself was responsible, or whether men who became committed partners and fathers started out with lower levels of the hormone than did their single, footloose friends.
To sort out cause and effect, anthropologists Lee Gettler, Christopher Kuzawa, and colleagues at Northwestern University in Evanston, Illinois, and the University of San Carlos in Cebu City, Philippines, checked testosterone levels in a group of men participating in the ongoing Cebu Longitudinal Health and Nutrition Survey. The survey began with a group of some 3000 women who were pregnant in 1983 and followed the general health, nutrition, medical care, and survival of their children; it has since expanded into an intergenerational study of health, education, and sexual behavior as those children grew up and are now having children of their own.
"The Cebu survey is one of the only studies of its kind, in which a large group of men has been followed since birth," Gettler says. "It also provides a changing culture, in which more fathers are getting involved with taking care of their kids, as opposed to the earlier tradition of being just breadwinners."
Working with a group of about 600 men participating in the survey, the team measured morning and evening salivary testosterone levels in 2005, when the men were about age 21, then again in 2009.
The researchers found that men with the highest levels of testosterone were more likely to become committed partners and fathers—at which point they showed steeper drops in testosterone than did their single, childless counterparts. New fathers showed a 26% drop in morning levels and a 34% decrease in evening levels, compared with single nonfathers, whose morning and evening testosterone went down by 12% and 14%, respectively (a decline attributable to the passing years). The study also revealed that testosterone levels were lowest in men who reported spending the greatest amount of time spent caring for their children. Low levels of the hormone also correlated with the age of the children, with the sharpest declines seen in men whose youngest child was less than a month old, the team reports online today in the Proceedings of the National Academy of Sciences.
"The drop in testosterone seems to be a biological adjustment that helps men shift their priorities when children come along," Kuzawa says. Other studies have found that men with high testosterone levels are more likely to have marital problems and get divorced. In one experiment, men with higher testosterone levels felt less sympathy and less need to respond to the sound of a crying baby.
By showing that committed fathers are able to lower their hormonal thermostats, the finding upends the classic view that men evolved solely to be hunters and providers, Kuzawa explains. "There's a growing awareness among anthropologists that raising human children is a group activity and that fathers are biologically wired to help out."
The paper is the first to show that committed parenting precedes lowered testosterone levels rather than being a result, says anthropologist Peter Gray of the University of Nevada, Las Vegas, writes in an e-mail. "This study addresses the chicken/egg issue head on."
"It's a very exciting study," adds psychologist Sari van Anders of the University of Michigan, Ann Arbor. "The researchers have done a solid job of demonstrating the evolutionary importance of fatherhood as an aspect of masculinity."