The blockbuster findings of a study published today delivered news that a friend of mine called “every father’s worst fear:” Testosterone levels nose dive once a man becomes a father and — this is the key — dip further when the father is a hands-on parent.

According to the study, testosterone levels declined by more than double once a man fathered a child. If that man reported spending more than three hours daily in caring for the child, the levels dropped lower still.

Published in the Proceedings of the National Academy of Sciences, the study looked at the hormone levels of 600 men at about age 21, all single and childless, and then again at the levels in those men five years later.

“ ... these findings show that T and reproductive strategy have bidirectional relationships in human males, with high T predicting subsequent mating success but then declining rapidly after men become fathers. Our findings suggest that T mediates tradeoffs between mating and parenting in humans, as seen in other species in which fathers care for young.” — from “Longitudinal evidence that fatherhood decreases testosterone in human males

If this study reported such hormonal changes in women, few would blink. It’s not news that women’s bodies respond in drastic ways to childbirth and child-rearing. Culturally, if not scientifically, we have been far less accepting of how a man is affected by becoming a parent.

There is evidence that men can suffer from postpartum depression, for instance, but very few men openly talk about it.

Fundamentally, we have the idea that testosterone equals power. To follow this logic, child-rearing could now be considered kryptonite. That’s the “worst fear” scenario.

Another way to look at it is that this study actually debunks the cultural myth that women are natural caregivers and men natural hunter-gatherers. Just as a woman’s body changes to accommodate children, so do men’s. Testosterone is supposed to dip. A father is supposed to be involved.

As Peter Ellison, a professor of human evolutionary biology at Harvard, told the New York Times:

“The real take-home message [is that] male parental care is important. It’s important enough that it’s actually shaped the physiology of men... My hope would be that this kind of research has an impact on the American male. It would make them realize that we’re meant to be active fathers and participate in the care of our offspring.”