Testosterone shrinks when men become dads. Does that mean they're designed to nurture kids?

For years, we've been told that men and women evolved for different roles. Men hunted, fought, and prowled for sex, leaving women to raise the kids. Now we're being told a different story: Men, like women, are designed to nurture children.

It's a nicer story, but it's just as simplistic. And the evidence is just as ambiguous.

The putative smoking gun for the new theory is a study just published in Proceedings of the National Academy of Sciences. It found that testosterone levels in young Filipino men fell 30 percent after they became fathers—more than double the reduction that occurred in childless bachelors of the same cohort during the same four-year period. There was a particularly big drop among men with infants. Furthermore, men who said they took care of their kids for at least three hours a day registered 20 percent less testosterone than did men who weren't involved in child-rearing.

The study has inspired at least half a dozen politically correct interpretations. Let's examine them.

1. Fatherhood awakens men's "nurturing side." According to Bloomberg, the study "suggests family men experience a biological shift that may awaken their nurturing side." The British Press Association reports: "Lowering testosterone is likely to make a man a better father by helping to bring out his nurturing feminine side, the study suggests."

Nurturing side? Sorry, there's no evidence. The only thing measured was testosterone.

2. Fatherhood makes men attentive to kids. So says an expert quoted in the New York Times. But attentiveness, like nurturing, wasn't measured. A different study indicates that men with lower testosterone are more sympathetic and responsive to babies' cries. But that study didn't measure changes over time.

3. Testosterone reduction makes men better dads by calming them. According to the International Business Times, "Lower levels help them become calm, attentive and evolve as a better father, the researchers found."

The researchers found no such thing. They didn't measure tranquility.

4. Men are designed to help raise kids. "Fathers Wired to Provide Offspring Care," says the press release from the study's authors. "Our study shows that human fathers are biologically wired to help with the job."