Having both parents care for a child not only increases its survival, but also improves its mental health, social skills, and educational prospects. Obviously, it’s best for both parents to be involved. But not all dads are great ones. There are a lot of reasons why a father may not invest in his kids—he may work long hours, live far away, or just not care. In this week’s PNAS, researchers from Emory University raise another possibility: maybe bad dads are the product of a life history tradeoff.

From a biological perspective, life is all about passing on one’s genes, and there are different ways to do that; one way is to invest heavily in a few precious offspring to make sure they survive, while another is to invest a whole lot of time and energy in mating and hope something pans out. Do human males exhibit this tradeoff? To address this question, the Emory researchers turned to testicles.

Yep, you read that right—testicles. Testes size has been linked to reproductive effort in several species; big ones generally mean increased sperm production, which increases the likelihood of mating success. Perhaps men with large testicles would be naturally predisposed to sow their seed rather than stay home with the kids. Guys who are less well-endowed, then, might be more likely to be the nurturing sort.

To test this hypothesis, the researchers rustled up 70 fathers of one- and two-year old kids and looked at a few different measures of parental effort and reproductive biology. First, they gave each dad a questionnaire to determine how involved they were in taking care of their child (these surveys were also given to the kids’ moms in order to get a second opinion on the matter). Then, the researchers took blood samples to analyze each dad’s testosterone levels. Finally, they put the dads in MRI scanners and measured the volume of their family jewels.

The researchers ended up using a measure called “residual testes volume,” which was the size of the guys’ testicles after controlling for their height. Testosterone levels and residual testes volume were moderately (but not completely) related, so the researchers kept both measures.

Their hunch was correct; well-endowed men were less likely to play a significant role in their kids’ care, according to their answers to the survey questions. Guys with smaller testes were more likely to take their children to the doctor, give them baths, and wake up in the middle of the night to rock them back to sleep than those with larger ones were. Testosterone levels, too, were correlated with parental investment; men with low testosterone were more nurturing than those that had high levels of testosterone coursing through their bloodstream.

But, of course, maybe these findings are the result of different types of men providing for their kids in different ways. Perhaps guys with hefty testicles and those with high testosterone levels tend to have high-powered careers and can’t spend time at home caring for the kids. Instead of doing the day-to-day care, maybe they are investing in their children by bringing home big salaries.

However, the researchers found that this wasn’t the case. Guys with large testicles or high testosterone didn’t tend to work longer hours or make more money than their smaller-than-average or low testosterone counterparts. Additionally, they didn’t want to invest as much in their children; men with large testicles reported significantly lower desired levels of involvement in their kids’ care.

However, questionnaires are subjective. To get a more objective biological assessment of the dads’ nurturing instincts, the researchers put each man in a fMRI machine and showed them pictures of their children. As the guys looked at these images, the machine measured the activity in the ventral tegmental area (VTA), an area known to be associated with caring behavior. Again, residual testes volume was correlated with nurturing thoughts; men with smaller testes showed much more activity in the VTA than those with larger endowments, suggesting that the smaller a man’s testicles are, the more nurturing his thoughts. Interestingly, testosterone levels were not significantly correlated with VTA activity.

It does appear as though there’s a link between testicle size and the tendency—as well as the desire—to invest in one’s kids. Of course, the relationship demonstrated in the study is correlational rather than causational. Testicle size in some species does fluctuate, and we all know that testosterone levels aren’t constant, so it’s possible that behavior affects biology here rather than the other way around. And the researchers didn’t assess or include the men's promiscuity, which would be a logical next step. But in any case, the size of a man's testicles does appear to be a relatively reliable indicator of his tendency to nurture and care for his children.

Of course, this doesn’t mean that all men with smaller-than-average testes are automatically great dads, nor are guys endowed with giant ones destined to be deadbeats. These results don’t give anyone an excuse or a reason to relinquish responsibility. However, they do suggest a more intricate relationship between our biology and behavior that merits a closer look.

PNAS, 2013. DOI: 10.1073/pnas.1305579110 (About DOIs).