Fatherhood Cuts Men's Testosterone

Dads who spend time with their children have lower levels of testosterone than single guys, according to a study that suggests family men experience a biological shift that may awaken their nurturing side.

While higher testosterone is considered beneficial for finding a partner, the male virility hormone drops once a stable relationship is formed and slides again during child-rearing, according to the study, published Tuesday in the Proceedings of the National Academy of Sciences. Researchers examined more than 600 men in the Philippines over about a five-year period.

The findings add to evidence that human males have evolved in ways that “facilitate their role as fathers and caregivers as a key component of reproductive success,” the researchers said in the study. Testosterone appears to mediate the tradeoff between mating and parenting in humans, they said.

“The men with the highest testosterone levels were more likely to become fathers by the time we followed up with them five years later,” said Christopher Kuzawa, a study author and an associate professor of anthropology at Northwestern University, in an interview. “Once they became fathers, their testosterone plummeted, quite a bit, more than any of the other groups.”

Testosterone is a hormone that promotes the development of male reproductive tissue and helps muscle, bone, and hair growth. Previous studies have shown that testosterone levels at the high end of a normal range are associated with lower amounts of blood fats and cholesterol, obesity, glucose intolerance, and diabetes. The abuse of testosterone, which can be taken as a supplement, is linked to increased risks of cardiovascular disease and sudden death.

Tracking Changes

Researchers first measured testosterone of single men in their early 20s who didn’t have children in 2005, then followed up 4 1/2 years later. Participants who reported during the later interview spending three hours or more with their children during the day had the largest declines in hormone levels, the study found.

Previous studies have shown that fathers in general have lower testosterone levels than single men without children. The prior research didn’t determine if men who started with lower levels of the hormone were more likely to enter into stable relationships and have children. The new study showed the opposite, Kuzawa said.

“There’s a lot of folk ideas out there about what testosterone is all about, but in the scientific community there’s much less certainty about this,” he said. “It’s not clear for instance that the decline in testosterone that we’re showing is going to influence someone’s libido, or have major effects in a lot of the things that men would care about.”

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