Testosterone levels plunging? Blame it on fatherhood, according to research from Northwestern University that found that the male hormone drops in men once they have children.
The hormone dips most precipitously in men who throw themselves into the details of child rearing, a finding that helps make the case that women aren't the only ones who are biologically wired for raising children. Decreased levels of testosterone may play a role in helping a man respond more sensitively to his children's needs and those of his entire family. "If this weren't something that had been normative in humans for the last 100,000 or more years, there would be no reason to expect this decline in testosterone," says the study's lead author, Lee Gettler, a biological anthropologist and doctoral candidate in Northwestern's department of anthropology.
Researchers collected testosterone via saliva samples from 465 Filipino men when they were 21 and single, then repeated the same tests when the men were 26. The men are part of a 28-year project that has followed the cohort since they were in the womb.
Men who remained single over the five-year span experienced a modest and expected age-related decline in testosterone. In men who became fathers, hormone levels dropped twice as much, according to the study, which was published in September in the Proceedings of the National Academy of Sciences. Men who partnered but did not father children fell in the middle.
The fathers who reported spending more than three hours a day caring for their kids had the lowest levels of all. "It really seems that the extent to which fathers end up with low testosterone is based on how actively they participate in child care," says Gettler. "It's not just that when men become fathers, their testosterone goes down. When they feed and bathe and play with [their children], it goes down substantially more. This seems to be an evolved trait — fathers caring for their young."