Researchers from Emory University have discovered that fathers with smaller testicles are more likely to be involved in caregiving activities like diaper changes, feeding, and nap time. Brains scans also show higher activity in their reward system. But the study is far from complete in its assessment.
It seems like a strange study to conduct, but the analysis was an effort to test the Life History Theory — the suggestion that an evolutionary trade-off exists between mating and parenting effort. The theory essentially asks, “Where is the bulk of a male’s time and energy better spent? Mating (i.e., sperm competition) or helping to rear offspring?” If affirmed, the Life History Theory could go a long way in explaining why some dads are more involved than others in the nurturing of their children.
To that end, a research team led by anthropologist James Rilling measured certain aspects of reproductive biology that relates to mating effort. The team also considered brain activity and how it relates to paternal caregiving behavior.
Indeed, as Sarah Zhang of Nature News points out, studies on other primates show that an apparent correlation exists between testicular size and caregiving effort. “[M]ale chimpanzees, which are especially promiscuous, sport testes that are twice as big as those of humans, make a lot of sperm and generally do not provide paternal care,” she writes. “By contrast, male gorillas have relatively small testes and protect their young.”
The new study, which appears in the Proceedings of the National Academy of Sciences, suggests that this is true for humans as well.
For the study, Rilling and colleagues looked at the relationship between testicle size and fatherhood in 70 men with children between the ages of one and two. Both the dads and the moms were asked to fill out surveys that assessed the father’s commitment to child care. The researchers also looked at brain scans while the men were told to look at pictures of their children.
The researchers learned that men with smaller testicles and lower levels of testosterone (measured by testes volume and plasma testosterone levels) were positively correlated with more involved paternal caregiving. MRI scans showed a three-fold difference between the volumes of the smallest and largest testicles in the group.
As for the brain scans, men with smaller testicles showed more activity in their ventral tegmental area — a component of the mesolimbic dopamine reward and motivation system.
So why the link? The researchers aren’t entirely sure. But they suspect that testosterone may have something to do with it. Other studies have shown, for example, that testosterone levels go down when men become involved fathers.
Needless to say, while somewhat revealing, the study is problematic on a number of levels.
Cultural and societal expectations on the role of the father were not accounted for in the study. Nor was socioeconomic status considered, the role (or presence) of an extended family, or the father’s prior history as a caregiver (e.g., did some men help to raise younger siblings?).
What’s more, the researchers didn’t go very far when choosing subjects for the sample pool, limiting their volunteers to men from the Atlanta area only. This is especially problematic in that it represents a very limited socio-cultural sample pool.
In all, the study provided, at best, a weak link indicating a male biological predisposition to nurturing behaviors. The jury is still out on whether it’s biological or societal influences that make a good dad.
Read the entire study at PNAS: “Testicular volume is inversely correlated with nurturing-related brain activity in human fathers.” Other sources: BBC and Nature News.