A new study, reporting that “better” fathers have smaller testicles, is creating a stir, probably in part for the number of jokes it inspires. Time’s headline is “Study: Choose dads with smaller ‘nads.” CNN commented on the fittingness of the study’s publication in the journal known as PNAS (say it in your head and you’ll get the joke), the nickname for Proceedings of the National Academy of Sciences. But there’s an actual point to the study: Men make “decisions” about where to put their energy resources – into “wooing” or into parenting – and testes size does seem to have something to do with it. The problem is that the study defines being a “good” father rather narrowly, so the study’s relevance to real life is completely up for grabs.
Studies in other primates have found that testicle size is linked to different “lifestyles”: Smaller testes are often seen in species whose males are more involved in the offspring, like gorillas, whereas larger ones are seen in the more “promiscuous” species, like chimpanzees. So the authors of the current study, out of Emory University, wanted to see if the same trend is seen within a single primate species: Humans. And since men who are more involved in their kids’ lives and development tend to have kids who are more successful in several realms, the study does have some larger sociological relevance.
In the study, the researchers had 70 fathers of kids one to two years old look at pictures of their own children, as well as unfamiliar children and adults, while undergoing MRI scans. The wives of the fathers also filled out questionnaires to rate how involved the dads were in raising their kids – for example, how often they changed diapers, stayed home from work if the child was sick, and fed and bathed the child. Testicle size and testosterone levels were measures to see what connections might be.
It turned out that men who were more involved in their kids’ upbringings had smaller testicles and lower testosterone levels.
What’s more, an area of the brain called the ventral tegmental area (VTA), which is thought to be involved in parental motivation, was activated more in these men. “The men with smaller testes were activating this brain region to a greater extent when looking at photos of their own child,” says study author Jennifer Mascaro, who did the research as part of her post-doctoral training.
Therefore, the authors speculate that testosterone might be linked to the “precopulartory” (or “wooing”) phase, but that testes size is linked to life after sex – life as a parent. In fact, testicle size seems to be more about sperm-production than testosterone production, since most of the interior of testes is made of seminiferous tubules, which pump out sperm. Therefore, this inverse relationship represents a “trade-off between spermatogenesis, a form of mating effort, and parental care.”
But as with any study, there are caveats, and in this one there are a lot.
First, it is a correlation, and it wasn’t a perfect one, which means men aren’t off the hook when it comes to parental involvement. “The fact that we found this variance suggests personal choice,” study author James Rilling says. “Even though some men may be built differently, perhaps they are willing themselves to be more hands-on fathers. It might be more challenging for some men to do these kinds of caregiving activities, but that by no means excuses them.” Practicing any behavior can often overcome a natural disinclination toward it, and parenting is no exception.
The other point to keep in mind is that there’s no evidence that testes size actually causes good or bad parenting behavior – in fact, it could very well be the opposite. Maybe the act of being a very involved father results in these changes in physiology. ”We’re assuming that testes size drives how involved the fathers are,” Rilling says, “but it could also be that when men become more involved as caregivers, their testes shrink. Environmental influences can change biology. We know, for instance, that testosterone levels go down when men become involved fathers.” The same could be true for testicle size.
The other big caveat, as the authors point out, is that only direct forms of childhood care were studied – like diaper-changing. Other forms, which may be even more important – like protecting a child in various ways, earning an income to give them better life, coaching little league, or teaching life lessons – were not studied. It could be that all of these things, which are hugely important aspects of caring for a child, could have other physiological correlates from what was seen in this study. In other words, we have no idea whether being a “good” father as defined in this study really matters in the grand scheme of things.
So, if your spouse has big testicles, don’t despair. The nature vs. nurture debate is often fruitless, and life is more about how the two work together than how either one predicts our behavior. While the study gives some hints as to how biology and psychology interact, the last thing we need is another measurement to worry about, especially at the expense of learning how to be a good parent.