Does it? The study shows that men are wired to produce less testosterone after their kids are born. There are several reasons, apart from caring or helping, why this might make evolutionary sense. For example:
A. Lower testosterone is healthier. As the authors note, testosterone is associated with heart disease and some cancers. That's reason enough for nature to lower your level once you've achieved the testosterone-assisted goal of fathering a child.
B. Lower testosterone reduces risky behavior. The authors cite substance abuse as an example. This is a definite advantage to your offspring, and it requires no helping or caring. You don't have to be Wonder Dad. Just don't get stoned all the time, take your kid hunting for wild pigs, or leave her in a hot car with the windows rolled up.
C. Lower testosterone reduces aggression. As a general rule, the most dangerous person in any household is the father. If you swaddle and play with your child, that's great. But first things first: Don't kill him or beat up your pregnant wife. Testosterone reduction is a good way to avoid this.
None of these alternative theories explains why spending more time with your kids lowers your testosterone more than just fathering a child does. But they don't have to explain that effect, because the study doesn't show it. In this paper, the authors report each man's testosterone level at two points: before and after he became a father. They don't report his testosterone before and after spending time with his kids. So while it's true that fathers who were more involved in childcare had lower testosterone than fathers who didn't, there's no longitudinal evidence that the childcare caused the lower testosterone, rather than the other way around.
The authors discount the possibility that lower testosterone causes paternal childcare, since pre-fatherhood testosterone levels didn't predict subsequent childcare involvement. But testosterone levels dropped radically as soon as the men became fathers. What was each man's testosterone level at that point? Without that information, we can't know whether lower testosterone preceded and predicted his degree of child care.
What we do know is that the correlation between testosterone and child care wasn't linear. Men who provided more than three hours of child care a day had more testosterone in the morning than did men who provided one to three hours. Men who provided one to three hours had more testosterone in the evening than did men who provided less than an hour. So the connection is unclear. And in a separate draft paper based on the same study population, the authors concede that their experiment to validate an effect of parent-child interaction on testosterone failed: "We hypothesized that T would decrease after men interacted with their children but found that T did not change during a 30-minute father-child interaction."
5. Fatherhood reduces infidelity. "Becoming a dad causes a testosterone drop that makes men less likely to stray," says the headline in London's Daily Mail. The paper reports: "Scientists have discovered fatherhood can halve testosterone levels—and stop men from straying." The Press Association agrees: "Men are less likely to stray after they have children as fatherhood lowers their testosterone, a study has shown."
The study shows no such thing. It didn't measure infidelity.
6. Fatherhood saps libido. "Children reduce dads' sex drive," says the headline in the Independent. The Mirror adds: "Men lose their sex drive when they become fathers so they can focus on their child." As evidence, the Mirror claims that the new study "found that newly partnered men who did not become fathers had similar sex drive levels to single men."
Rubbish. Testosterone and sex drive are sometimes related but never equivalent. "It's not clear … that the decline in testosterone that we're showing is going to influence someone's libido," one of the study's authors tells Bloomberg. To the Times, he points out: "You don't need a lot of testosterone to have libido." The study bears this out: Post-fatherhood declines in testosterone didn't stop the men from having more kids.
Testosterone affects and is affected by many things. It probably does adjust to environmental cues as men become mates and then fathers. But we're just beginning to explore how and why this happens. The new evolutionary-psychology theory we're being fed has less to do with earth-shattering evidence than with changes in our economy and culture. Women are gaining more respect and consideration. Wages have shrunk, so both parents have to work for pay. Men have to help out more at home, and they can't get away with cheating the way they used to. For a bunch of reasons, we need a more domestic and egalitarian theory of masculinity. And we're using this study to sell it.