"Daddy, why's your testosterone falling?"

Fathers often struggle to describe how becoming a dad transforms them in profound and moving ways.  "I feel more connected ... more grounded ... more sensitive ... just different, you know?"  Changing diapers, pureeing carrots, backpacking the baby through local parks, reading The Little Engine that Could until he can't leads Dad to believe that his very essence has been altered.

Thanks to researchers at the National Academy of Sciences, now we know it has.  The results of a study following more than 400 fathers shows that testosterone, the King of male hormones, actually decreases after a man becomes a parent.  And the more involved he is as a parent, the steeper the decrease in testosterone.

The implications of these findings are enormous.  Contrary to those who deny any family role for men other than insemination, this report confirms a biological foundation for fathers-as-nurturers as well as a sociological imperative for society to radically alter its expectations of dads.

In the 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, reading or dressing them – had the lowest testosterone.

It looks likes the Daddy Gene has been found.

"The real take-home message" of this study, said Peter Ellison, a professor of human evolutionary biology at Harvard University interviewed by the New York Times, "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," Ellison concluded.  "It would make them realize that we're meant to be active fathers and participate in the care of our offspring."

Of course one worry is that men will see that active fathering lowers testosterone and head for the hills – or the nearest pick-up bar.  But have no fear – testosterone levels lowered by active fathering isn't irreversible. One study of Air Force veterans shows  that testosterone climbed back up after men were divorced.  Others show that an elevated testosterone level isn't a requirement for an active libido, welcome news to sexually active moms and dads alike.

So to those fathers pulling down major time with your kids: it's clear that's exactly what you're made for.  And to those not involved, and to divorce courts that customarily cut fathers out of their children's lives by limiting them to "visitation rights" – get with the plan and keep dad in the game.  It's good for everyone.

“Humans give birth to incredibly dependent infants," said Lee Gettler, an anthropologist at Northwestern University and co-author of the study.  "Historically, the idea that men were out clubbing large animals and women were staying behind with babies has been largely discredited. The only way mothers could have highly needy offspring every couple of years is if they were getting help.”

So do not ask for whom the testosterone lowers – it lowers for the kids.  And that's a good thing.