“The real take-home message,” said Peter Ellison, a professor of human evolutionary biology at Harvard who was not involved in the study, is that “male parental care is important. It’s important enough that it’s actually shaped the physiology of men.”

“Unfortunately,” he added, “I think American males have been brainwashed” to believe lower testosterone means that “maybe you’re a wimp, that it’s because you’re not really a man. 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.”

The study, experts say, suggests that men’s bodies evolved hormonal systems that helped them commit to their families once children are born.