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Tuesday, December 14, 1999Men's hormones change to make them better fathers: study
Less testosterone, more nurturing
Men undergo significant hormonal change during their partner's pregnancy as a kind of last-minute dose of paternal instinct, Memorial University researchers have found.
In a paper to be published early next year in the journal Evolution and Human Behaviour, the psychologists report that as the delivery date nears, expectant fathers experience surges in their levels of prolactin and cortisol -- two hormones associated with the onset of parental behaviour.
The study also found that men's testosterone levels decrease by one-third immediately after they become fathers, providing a calming effect that may make them less likely to stray from home.
"It appears that the testosterone decrease in the postnatal period may enhance paternal responsiveness in men by reducing their tendencies to engage in incompatible non-nurturing behaviours," the authors write.
Anne Storey, a psychology professor at Memorial and the paper's lead author, said she began the study three years ago as a follow-up to her research on the parenting behaviour of sea birds and rodents. Women's hormonal changes during pregnancy are well-documented, and animal studies have shown hormonal changes in males linked to their mates' pregnancy. But Dr. Storey said these are the first published results showing similar effects in men.
The study followed 34 couples recruited from prenatal classes offered by Grace General Hospital in St. John's. All but three of the couples were first-time parents. They provided blood samples throughout the pregnancies.
In the men, levels of prolactin, the hormone responsible for producing breast milk, increased by about one-third between the middle and end of the pregnancy. The levels were still well below those in lactating mothers, but the researchers say the prolactin boost could make a father more responsive to his infant even if he is not much use in the feeding department.
The men's cortisol levels doubled by the late-pregnancy stage. Cortisol in women is involved in the onset of labour and it has been linked to mothers' attachment to their newborns. For example, an earlier study found that new mothers with higher cortisol levels were more attracted to infant odours.
The researchers suggest that the higher cortisol levels in men "help new fathers focus on and become attached to their newborns." They conclude that there are "strong parallels" between the hormonal changes experienced by men and women during pregnancy.
"If the same pattern of changes is also taking place in the male, maybe that's also helping them get up to speed more easily when their babies are born," Dr. Storey said in an interview. "If the dad's going to be involved, it's a sensible mechanism."
She said the precise mechanics of the changes experienced by men are unclear. It appears the man's hormonal changes closely mirror his partner's, and one theory is that "pheromonal communication" -- involving the sense of smell -- could be responsible.
Only couples living together took part in the study, but the hypothesis is that fathers who do not live with their babies' mothers would not undergo the same changes. "There are probably several ways that we become good parents, but maybe this process of being together during the pregnancy just helps people along a little bit," Dr. Storey said.
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