Levels of the male sex hormone drop sharply after men become fathers, a biological change that suggests men are hard-wired to care for their children, according to researchers studying human testosterone.
Previous studies have found that fathers have lower testosterone levels than non-fathers. Northwestern University biological anthropologist Christopher Kuzawa says there are at least two possible explanations.
"Is it that fatherhood reduces your testosterone?" he asked. "Or do men with low testosterone to begin with, are they more likely to become fathers? And so what we did is, we followed men through time and measured their hormones before and after they became fathers."
Kuzawa and his colleagues tapped into a decades-long study of a group of men in the Philippines that measured participant testosterone levels in 2005 and in 2009.
During that time, some of the men were still childless, but others had become fathers.
"And those are the men where we see the largest decline in testosterone," says Kuzawa, who found that testosterone levels dropped by about half immediately after the birth of the child, then rebounded somewhat. Men who were actively involved in caring for their children produced less testosterone than men who were not involved.
It might be that men are programmed to have lower testosterone levels, so they can focus more on parenting than procreating. But Kuzawa says the hormone reduction carries with it some health benefits, too.
"Having high levels of testosterone can increase your risk for diseases like prostate cancer [and] testicular cancer. Also, testosterone can suppress the immune system so that you're less capable of fending off pathogens."
Kuzawa's study is published online in the Proceedings of the National Academy of Sciences.