Does The Size Of A Man’s Testicles Say Anything About His Parenting?

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A new study looking at the link between men’s testicular size and their parenting qualities has been making the rounds in the media lately, with provocative headlines ranging from “Choose Dads with Smaller ‘Nads” to “Dudes with Smaller Balls are Better Parents” to “Big Testicles Indicate Rubbish Dads.” The message is pretty clear: if you’re looking for a baby daddy who’s going to stick around and take care of your kids, look for the guy with the smallest set of testicles you can find. However, before you go and dig out the measuring tape, it's worth taking a closer look at the details of this study.

Researchers recruited 70 men who were the biological dads of a one- or two-year-old child and who were living with the child’s mother (93% were married to the mother). All men were between the ages of 21 and 55 and did not have any significant health history. Using an MRI machine, the men had scans of their brains and testes taken. A blood test was also performed to assess testosterone levels.

While inside the brain scanner, the men were shown pictures of their own child, as well as photos of a few unrelated children. The researchers wanted to record how much brain activation occurs in the ventral tegmental area (a part of the brain’s reward and motivation system) when seeing one’s own child. Based upon past research exploring this brain region, greater activation was theorized to be a sign of nurturing tendencies and a drive to care for one’s own children. Next, in order to measure parenting contribution, the researchers had the female partners complete a questionnaire about the role the fathers play in caregiving (e.g., how often they participate in bathing the child, taking the child to the doctor, etc.).

Results indicated that testicular size was inversely related to caregiving, meaning that men with smaller testicles took responsibility for more caregiving. Levels of testosterone (the primary sex hormone produced by the testes) showed the same association (i.e., low testosterone was related to more caregiving).

Testicular size was also related to the amount of brain activation observed in the ventral tegmental area when shown photos of one’s own child. Specifically, the smaller a man’s testicles, the more nurturing-related brain activation he showed. However, this increase in brain activation was not specific to seeing one’s own child—in fact, seeing photos of any child was linked to greater brain activation among men with smaller testes. In other words, these men showed greater neural “responsiveness” to all children. In contrast, testosterone levels were unrelated to patterns of brain activation.

The authors of this study interpret these findings as support for Life History Theory, which argues that limited resources require humans to make a trade-off between mating efforts and parenting efforts. We’re not optimized for doing both, so some humans are optimized more for one than the other, which is thought to be advantageous for the survival of our species (i.e., if we all became too focused on mating, our parenting would suffer and that would hurt the survival of our offspring, while if we all became too focused on parenting, we would mate less and our population would dwindle). So, men with bigger testicles and more testosterone are thought to be evolutionarily designed for mating, while men with less bulk in their scrotums and less testosterone are thought to be designed for parenting. I don’t know if I buy it, but that’s the basic idea.

So if you’re looking for a reliable father, does that mean you should start screening men based on the size of their testicles? That probably wouldn’t be the wisest move. For one thing, keep in mind that we’re dealing with a single study on a very small sample of men here, all of whom were from just one city in the U.S. (Atlanta). Moreover, guys who are willing to have their scrotums x-rayed for science may not be representative of the rest of the male population. At the very least, we would want to see whether these findings hold up in other studies with more diverse samples.

More importantly, however, this is a correlational study, which means we cannot say whether testicle size causes more involved parenting. Although this is an unlikely alternative explanation, it could be that highly involved parenting or nurturing tendencies affect testicle size. A more plausible alternative is that guys with larger testicles are socialized differently. As some support for this idea, consider that this study found that taller guys had larger testes, and we know from other research that taller guys are perceived more positively and receive more favorable treatment (e.g., they make more money). In other words, testicle size isn't the only physical difference between these men, and those physical differences may be related to psychological differences (e.g., having a bigger ego). Yet another possibility is that men with larger testicles still invest in their children, but just not in ways captured by the caregiving measure that was administered (in other words, maybe the media was too quick to call guys with big testicles “rubbish dads," given that the researchers didn’t measure all aspects of parenting in this study).

In short, there are a lot of other things that could be going on and we do not know which explanation is correct, so you probably don't want to waste too much time pondering the deeper meaning behind the size of a man’s scrotum.  

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To read more about this research, see: Mascaro, J.S., Hackett, P.D., & Rilling, J.K. (in press). Testicular volume is inversely correlated with nurturing-related brain activity in fathers. Proceedings of the National Academy of Sciences.

Image Source: iStockphoto.com 

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