Professor Kuzawa is a biological anthropologist. He and other researchers used information from a long-term study of men in the Philippines. That study measured their testosterone levels in two thousand five and two thousand nine.

Some of the men were childless; others became fathers during that time.

CHRISTOPHER KUZAWA: “And those are the men where we see the largest decline in testosterone.”

The study found that testosterone levels dropped by about half immediately after the birth of a child, then rose some. Men who were active in child care produced less than those who were not active.

Testosterone levels may drop in fathers to get them to pay more attention to parenting than to reproducing. But Professor Kuzawa says there are other good reasons as well.

CHRISTOPHER KUZAWA: “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.”

The study is in the Proceedings of the National Academy of Sciences.

Studies have shown a similar drop in other mammals and in birds after mating periods to allow for care giving. Animal studies have shown that the greater the involvement of males, the lower their testosterone levels.

We asked Thomas McDade, another author of the study, if testosterone levels also drop in men who adopt children. He says that is an interesting question and the “next obvious thing to study.”