A man's testosterone levels fall when his child is born, researchers have found.

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Men who started with high testosterone were more likely to become fathers, but once they did, their levels dropped. (Lido Vizzutti/Flathead Beachon/Associated Press)

In this week's online issue of the Proceedings of the National Academy of Sciences, Lee Gettler, a doctoral candidate in anthropology at Northwestern University in Evanston, Ill., and his co-authors followed 624 young men in the Philippines to see whether their testosterone levels changed after becoming fathers.

"The men who started with high testosterone were more likely to become fathers, but once they did, their testosterone went down substantially," Gettler said in a release.

"Our findings suggest that this is especially true for fathers who become the most involved with child care."

Men who spent more than three hours a day changing diapers, giving baths and reading bedtime stories showed the lowest testosterone.

"Our findings suggest that testosterone mediates tradeoffs between mating and parenting in humans, as seen in other species in which fathers care for young," the study's authors concluded.

The men in the study were followed for about 4.5 years.

The research was funded by the National Science Foundation and the Wenner Gren Foundation.