Men’s testosterone levels drop after fatherhood, study finds

LOS ANGELES – Hormonally speaking, becoming a father may make you less of a man, according to new research that finds testosterone levels drop in men after they become parents. But fear not for manhood – the results show that men are evolutionarily wired to help raise their children and that testosterone may get in the way, scientists say.

Testosterone is a hormone associated with perceived hallmarks of masculinity such as libido, aggression and musculature. Those can be useful qualities when competing for a mate, but less so when raising a child – an endeavor that requires calm, attentiveness and an even temper.

Some studies over the last decade had shown that fathers have lower testosterone levels than their childless counterparts. But it remained unclear whether fatherhood itself was responsible for the decline or if men with less testosterone were more likely to settle down and become fathers in the first place.

To answer the question, scientists measured the testosterone levels of 624 Philippine men who were 21 years old. Researchers tracked the men – and their hormones – over the next 4.5 years.

The data revealed that the men with higher testosterone levels at the start of the study were more likely to become fathers. But these dads saw their testosterone levels plummet by 26 percent upon waking and by 34 percent at bedtime over the course of the study. Compare that to the bachelors, who saw modest age-related declines of 12 percent and 14 percent.

For fathers, the initial drop was abrupt: Men with newborns saw T-levels dive by 43 percent in the morning and 49 percent in the evening during the baby’s first month of life. As their infants grew, their hormone levels recovered – men with babies between the ages of 1 and 12 months took a testosterone hit of only 23 percent in the morning and 35 percent in the evening.

Overall, men who devoted the most time to child care had the lowest levels, according to the study, published online Monday by Proceedings of the National Academy of Sciences.

“There’s something about being an active father that’s contributing to these dramatic declines,” said study leader Lee Gettler, a biological anthropologist at Northwestern University in Evanston, Ill.

The results support the idea that testosterone levels aren’t static but respond to a man’s behavior and cues from his cultural environment, said Peter Ellison, an evolutionary biologist at Harvard University who was not involved in the study.

For what it’s worth, researchers suggested that a dad’s testosterone is likely to bounce back as his children mature and become self-sufficient.