Only a generation ago, science promised to unlock the seemingly ineffable mystery of love — and we were regaled with stories of the the effects of vasopressin on pair-bonded voles on the American prairie and imaginings of romance's origin among hominids on the ancient African savannah.

Now, researchers are exploring the biological underpinnings of something less mysterious, the guy who plays golf on weekends, chain smokes in the car, and has nothing good to say about politicians in Washington. The American Dad.

James Rilling, an anthropologist at Emory University who studies the neurological basis of human behavior, explained the biological transformation men make when becoming fathers in a pair-bonded union, whether married or simply living together under one roof.

"Mothers have an advantage, in that the hormones of pregnancy give them a head start and get them primed to be nurturing," he said. "In particular, when women give birth, there's a big surge of oxytocin, and oxytocin is also released during breastfeeding. Fathers don't have that."

But nature has its own tricks for turning the finely tuned hunting and killing machine into a nurturing father, intent on providing and protecting. A father's behavioral interactions with his children increases oxytocin levels similar to that seen in women. Other hormonal effects included reductions in testosterone, which promotes aggression and mate-seeking behavioral. Researchers found not to long ago that higher levels of the hormone vasopressin, which is also known as antidiuretic hormone in mammals, induces lifelong pair-bonding in prairie voles.

Also useful in suppressing a man's urge to seek other mates are elevated levels of the hormone prolactin, which is produced in men following sexual climax.

However, these hormonal changes only occur in men who spend time around their children, with much less pronounced effects in men separated from their progeny.

"There seems to be some kind of fundamental social-neurobiological framework that comes into play when fathers interact with their kids," Lee Gettler, an anthropologist at Notre Dame, told reporters. As for the different routes by which the male and female brain came to the same hormonal response, Gettler believes that nature took the easiest route.

"It may be that the most parsimonious way to engineer a paternal brain would be to take the circuitry that was already in place for maternal care, and maybe tweak that," he said. That might be the reason there's some overlap there."

Like the rolling prairies and their vasopressin-drunk voles, the neuroscience of parenthood — in the brains of both men and women — remains a "wide open frontier," Rilling said.