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After marriage, a man's testosterone level will drop significantly Add to ...

"They have their babies, but they are talking guy talk. It's just amazing to watch," says Dr. Hewlett, who has studied the Aka for 30 years since he was a graduate student and wrote a book about them called Intimate Fathers.

Even so, the Aka are less child-focused than many North American families, he says. The babies and kids are part of whatever activity their parents are doing -- but not necessarily the centre of attention. As well, the Aka never talk about their children as though they are burdens. They don't complain about sleepless nights or difficult toddlers.

Over the years, Dr. Hewlett says, his work with them has changed his own approach to being a parent. He has seven children from 15 to 25 years old, and tries to spend more with them and include them in what he is doing. "I try to take the kids to work."

Not that he believes hunter-gatherers like the Aka have all the answers for Western parents.

They don't, for example, use anything resembling diapers, and some experts have gone as far as to suggest that parents in the developed world also could learn to anticipate when their babies need to go, and hold them over an appropriate receptacle. But Dr. Hewlett says he has seen too many soiled Aka moms and dads to think much of this toilet-training tactic.

Working with men who spend so much time caring for their infants -- wet or dry -- also piqued his interest in the role hormones might play in fatherhood. He conducted a study in the United States in which he took blood samples from fathers before they held their infants, and again after they had them on their chests for 15 minutes. Their prolactin levels went up.

"There is a biology of fatherhood, but it is impacted by the environment," Dr. Hewlett says. "If you don't hold the baby, certain things don't get triggered."

It isn't clear what, exactly, triggers the physical changes in fathers, although animal research, including work on two species of primates, suggest that pheromones -- chemical signals produced by the mother -- could be the key.

But the system may not be so straightforward. Dr. Wynne-Edwards and her colleagues have lowered the levels of hormones in male hamsters, which normally act like superdads. But this hasn't changed their nurturing behaviour, which she believes may have been prompted instead by their own upbringing or by something that happens when they mate.

In humans, researchers have found that testosterone levels drop significantly after marriage, long before many men become fathers.

Ottawa's Mr. Charron doesn't know if his hormone levels have changed, but he does know that he is more emotional. "I'm a softie," he says. "The tears just come" -- often prompted by a tragedy in the news that involves children.

Spending so much time with Sophie may even have rewired his brain.

James Swain, a Canadian researcher at Yale University, used functional magnetic-resonance imaging to look at the brain activity of 25 couples after they had a baby. He and his colleagues took brain images of the new moms and dads when they heard the sound of their infant crying, or looked at a picture of their newborn.

They found that the brain activity of new parents is strikingly similar to what is seen in patients with obsessive-compulsive disorder when their symptoms have been triggered.

It is not that new parents are mentally ill, but they do tend to fuss. Dr. Swain's findings may help to explain why some moms and dads report constantly sneaking in on sleeping children to make sure they are still breathing, or adjusting infants' clothing or bedding to make sure they are not too hot or too cold.

This adaptive parental fussing seems to require some of the same brain circuits gone awry in obsessive-compulsive disorder. The effect was more pronounced in women two weeks after the baby was born, Dr. Swain says, but was clearly there for men as well.

By three months, however, the brain response of the father to the sounds of his baby crying was comparable to that of the mother.

"It may be that men are biologically primed or programmed, perhaps even by their own baby, to become attentive fathers," he explains. "This is measurable in their psychology and their brain activity in specific parenting circuits in the brain."

Findings like these intrigue Michael Lamb, a psychologist at Cambridge University who is considered the father of research on fathers.

Like many in his field, at one time he believed that mothers and fathers play distinct biological roles: Mom made sure the child was warm, fed and cuddled. The baby became attached to her through this regular care. Dad's department was rough-and-tumble play, a more boisterous and physical kind of stimulation. The baby bonded with him through play.

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