A new study shows that men who spend time with their children have lower levels of testosterone than single guys. The study shows that family men experience a biological shift that may awaken their nurturing side.
While higher testosterone is considered beneficial for finding a partner, the male virility hormone drops once a stable relationship is formed and slides again during child-rearing, according to the study, published today in the Proceedings of the National Academy of Sciences. Researchers examined more than 624 men in the Philippines over about a five-year period.
The findings add to evidence that human males have evolved in ways that “facilitate their role as fathers and caregivers as a key component of reproductive success,” the researchers said in the study. Testosterone appears to mediate the trade-off between mating and parenting in humans, they said.
“The men with the highest testosterone levels were more likely to become fathers by the time we followed up with them five years later,” said Christopher Kuzawa, a study author and an associate professor of anthropology at Northwestern University, in an interview. “Once they became fathers, their testosterone plummeted, quite a bit, more than any of the other groups.”
Researchers first measured testosterone of single men in their early 20s who didn’t have children in 2005, then followed up 4 1/2 years later. Participants who reported during the later interview spending three hours or more with their children during the day had the largest declines in hormone levels, the study found.
Previous studies have shown that fathers in general have lower testosterone levels than single men without children. The prior research didn’t determine if men who started with lower levels of the hormone were more likely to enter into stable relationships and have children. The new study showed the opposite, Kuzawa said. “There’s a lot of folk ideas out there about what testosterone is all about, but in the scientific community there’s much less certainty about this,” he said. “It’s not clear for instance that the decline in testosterone that we’re showing is going to influence someone’s libido, or have major effects in a lot of the things that men would care about.”
And the researchers believe lower testosterone levels might protect against certain chronic diseases, which could, in part, explain why married men and fathers often enjoy better health than single men of the same age. Experts say the new testosterone study could offer insight into men’s medical conditions, particularly prostate cancer. Higher lifetime testosterone levels increase the risk of prostate cancer, just as higher estrogen exposure increases breast cancer risk.
Prof Ashley Grossman, spokesman for the Society for Endocrinology, said life and biology may be “much more subtle and adaptable than we had previously thought.” “This shows the hormonal and behavioral trade-off between mating and parenting, one requiring a high and the other a low testosterone level,” he said.
Dr Allan Pacey, senior lecturer in andrology at the University of Sheffield, said the findings were fascinating, “Testosterone levels in men generally don't change that much. They can slowly decline as men get older and change in response to some medical conditions and treatment. But to see dramatic changes in response to family life is intriguing. The observations could make some evolutionary sense if we accept the idea that men with lower testosterone levels are more likely to be monogamous with their partner and care for children. However, it would be important to check that link between testosterone levels and behavior before we could be certain.”
The study did not examine specific effects on men’s behavior, like whether those with smaller drops in testosterone were more likely to be neglectful or aggressive. It also did not examine the roles played by other hormones or whether factors like stress or sleeplessness contributed to a decline in testosterone.
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