The male hormone, testosterone, takes a hit once men make the transition to fatherhood, suggesting it may be in a man’s biological makeup to provide direct care for his children once they’re born, according to a recent Northwestern-led study.
The research builds off a flurry of experiments within the past two decades that looked for a link between a man’s relationship status and his testosterone levels. Some of these past studies reported that fathers had lower levels of the hormone than non-fathers. However, NU researchers said because these studies only measured testosterone and fatherhood status at a single point in time, they could not prove whether having children caused these hormonal dips.
“Previous studies were unable to discern what was ‘chicken’ and what was ‘egg,'” said Lee Gettler,, a graduate student at NU and lead author of the study. “They showed fathers often had lower (testosterone) than non-fathers, but it was not clear if men with low testosterone were simply more likely to become fathers or if fatherhood itself somehow caused testosterone to decline.”
To clarify this correlation, researchers employed a longitudinal design in which they measured the testosterone levels of over 400 single non-fathers in their early twenties. The participants were tested again four years later, at which point they disclosed whether they were still single, in a relationship without kids, or partnered fathers. Of the three groups, partnered, first-time fathers showed the largest declines in testosterone levels. Additionally, researchers observed that the scale of the father’s involvement was an additional factor: the more time these new fathers spent taking care of their kids, the lower their testosterone levels were at follow-up.
Overall, these findings provide convincing evidence that a man’s experience of fathering and raising a child and reduced hormone levels are linked by causation – not by mere correlation, said co-author Chris Kuzawa, a professor of anthropology at NU.
“Our study provides the strongest evidence to date that male biology reduces testosterone in response to changes in relationship status and especially after becoming a father,” Kuzawa said. “We tend to imagine that hormones influence our behavior, but our study illustrates nicely how the opposite can also be true – our behavior and social environment have powerful effects on our biology and hormone levels.”
The study’s results may help revise the long-held view that men evolved solely for the purpose of hunting and providing for their families, only to leave childcare squarely within the purview of women. In fact, it demonstrates that from an evolutionary viewpoint, men may have been biologically “wired” for paternal care, Gettler said.
“It is somewhat ingrained that women are more nurturing and perhaps biologically designed for, or at least biologically prepared to be, parents,” he said. “I think our findings really suggest that men, in their own physiological way, have the capability to respond to the challenges of parenthood with appropriate biological responses.”
Additionally, it may be that having men respond with lowered testosterone levels works in the best interest of a couple’s offspring, said NU psychology professor Eli Finkel.
“(The study) suggests that men who have children exhibit hormonal reactions that are likely to decrease their interest in having sex with other women,” he said. “Such processes are likely to promote pair bonding with the mother of the man’s children, a process that has traditionally increased the likelihood of offspring survival.”
For now, the big question that remains for researchers is how testosterone drops following fatherhood. According to Kuzawa, future research should focus on understanding the mechanisms that drive this process.
