If you asked a normal father why he stayed around after his child was born he might be a bit offended, and then he might say that it was because he loved his child and its mother (his wife) and because, anyway, he is responsible, along with his wife, for the children they bring into the world. He might also point out that he committed himself to the family when he married his wife. In other words, he would give a moral explanation.

Much is being made this week, however, of a study that shows testosterone declines markedly in men who become fathers, giving them a push in the direction of committed parenthood. It’s, like, suddenly we can know that fathers are meant to be with their children because biology proves it. Oh well, anything that reinforces the message is welcome.

It seems that the more a father gets involved in caring for his children, the lower his testosterone level drops.

The study was done in the Philippines among 600 men who, says the New York Times, were part of a “larger, well-respected health study following babies who were born in 1983 and 1984”. (It would be interesting to find out more about the larger study.)

Testosterone was measured when the men were 21 and single, and again nearly five years later. Although testosterone naturally decreases with age, men who became fathers showed much greater declines, more than double that of the childless men.

And men who spent more than three hours a day caring for children — playing, feeding, bathing, toileting, reading or dressing them — had the lowest testosterone.

This has, naturally, given a boost to theories that dads should spend a lot more time in doing childcare. But how many men who are breadwinners (somebody has to be when children are young) could spend more than three hours a day caring for their children? Pretty well everyone agrees that fathers need to do some of the childcare and probably most want to, but there is an idea out there that the ideal is a half share.

Research by W Bradford Wilcox on women’s marital happiness, however, has shown that women can be happy with doing the bulk of childcare and housework while the husband does paid work, because they feel the arrangement is fair.

One benefit of hands-on fathering implied by the study is a lower risk of prostate cancer, because of lower long-term exposure to testosterone. The more kids, the better?

Finally, an interesting question: primatologist Sarah B Hrdy wonders whether only biological fathers are affected, or would similar results occur "if you have an uncle or brother or stepfather living in the household and they care for the baby?"

One answer: we know that the risk of abuse and neglect is highest for children living with their mother and her boyfriend (non-related male) especially when he is left to care for a child by himself. That sounds like an excess of testosterone at work.