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Northwestern University anthropologists found that the male sex hormone testosterone drops dramatically in men after they become fathers.
By msnbc.com contributor
updated 9/12/2011 3:07:26 PM ET 2011-09-12T19:07:26

Fathers may no longer have the excuse that they aren’t born for child care. Actually, they’re designed for it, according to a new study that finds a significant drop in testosterone levels in new dads, suggesting that men may be wired to nurture.

Northwestern University anthropologists speculate that the drop in the primary male sex hormone signals that fathers evolved to care for kids, not just to hunt for game and drop it in mom’s lap.

Their findings are published in the latest issue of the Proceedings of the National Academy of Sciences.

The anthropologists, led by doctoral candidate Lee Gettler, measured testosterone levels in more than 600 men in and around the large city of Cebu, in the Philippines, before the men married and then again after marriage and fatherhood. Fathers showed marked declines in salivary testosterone, with those who were both newly partnered and with newborns showing a dramatic drop of 34 percent from the levels taken when they were single.

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Generally, Gettler said “we found that when the men were 26 years old, those who were most involved [in childcare] had the lowest testosterone levels.”

While the findings might alarm prospective new dads already struggling with the switch from sports cars to minivans, experienced fathers say they don’t feel less manly.

New York City resident Lance Somerfeld, 38, a former school teacher, and founder of nycdadsgroup.com, stays at home to care for his 3-year-old son while his wife works. He doesn’t feel much different than he did before having his son, he said, though he’s pretty exhausted by the end of the day. He does notice that “Sunday afternoons, if I have a choice of meeting guys for fantasy football or spending an hour or two with my son, I usually choose to spend time with my son.”

Somerfeld describes his life as happy and content. “It’s in a good place.”

Contentment may be the point
That’s exactly the kind of thing a drop in testosterone accomplishes, suggest Gettler and co-author Christopher Kuzawa, an associate professor of anthropology at Northwestern. Human babies are dependent on parents for years. Even with a three-year spacing between children, mom needs help with the kids. When dad helps, his testosterone declines, making him feel content so he can focus on mother and children rather than, say, fighting a war, looking for sex, or playing fantasy football.

“There is evidence in several monogamous animal species that testosterone concentrations decrease once males become dads,” explained Larry Young of Emory University who studies the neuroscience of social bonding. (Note: I’m co-writing a book with him on the subject.)

But does a drop in testosterone act in the brain to bias fathers toward caregiving, or does caregiving itself lower testosterone?  

Research in some primates has shown that when a father sniffs the scent of his own babies, but not strange babies, his testosterone drops within minutes. This could be protective, lowering aggressive behavior so dad doesn’t get rowdy and mean around the kids and maybe attack them. In birds, a drop in testosterone with fatherhood appears designed to lower aggression and territoriality so dad spends time on his chicks.

“We did our best to disentangle” the which-comes-first question, Gettler said. He could find no relationship between testosterone levels when the men were single and later caregiving involvement, leading him to think the act of nurturing lowers testosterone.

While it’s not proven, that sounds right to Young. “Many studies in mammals question whether the drop in T has a causal role,” he explained. “The most likely relationship is that the experience of being a father, interacting with and caring for offspring, simply suppresses the secretion of testosterone. This may shift the emotionality of the father toward a more nurturing state of mind.”

Because other studies have shown that marriage itself lowers testosterone in men, it could be that falling levels may be nature’s way of “taming” men so they can adapt to family life. 

Follow Brian Alexander on Twitter: @BrianRAlexander

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