What makes some fathers get up in the middle of the night to comfort their crying baby while others sleep right through? The size of their testicles may be a factor, a new study suggests.

The smaller a man’s testicles, the more involved he is likely to be in caring for his toddler son or daughter, suggests a study conducted by anthropologists at Emory University in Atlanta, Ga., and published Monday in the Proceedings of the National Academy of Sciences.

The researchers, led by Jennifer Mascaro, used an MRI to measure the testicles of 70 Atlanta-area fathers who had a child between the age of one and two and lived with the child’s mother as a family. The fathers and their partners had previously been asked a series of questions about who in their family was more likely to perform each of 24 parenting tasks such as changing diapers, bathing their child, take them to medical checkups or attending to them when they woke up in the middle of the night.

By scanning the men’s brains with an fMRI machine while showing them photos of their children, the researchers found they could predict which fathers would be more involved in their children’s care. Men who showed more activity in a part of the brain known to motivate animals to nurture their offspring performed more hands-on parental caregiving.

In general, smaller testicles were also associated with higher nurturing-related brain activity, suggesting that men with smaller testicles were more biologically motivated to care for their children.

Mating versus parenting

Larger testicles are linked to higher sperm counts and therefore the amount of biological effort put into mating. The researchers said their results therefore suggest there is a biological trade-off between the amount of effort put into mating versus parenting in humans.

On the other hand, the study didn’t pinpoint the reason for the link between testicle size and parenting effort.

"We're assuming that testes size drives how involved the fathers are," said James Rilling, a co-author of the paper and the head of the lab where the research was performed, in a statement, "but it could also be that when men become more involved as caregivers, their testes shrink."

That would be similar to what happens with testosterone levels, which fall when men become fathers — something that also happens in other animals where the fathers are active parents. In fact, the researchers chose to focus on testicle size rather than testosterone levels because hormone levels tend to fluctuate much more over time.

The researchers also noted that while the correlation between testicle size and caregiving was statistically significant, it wasn’t perfect.

"The fact that we found this variance suggests personal choice," said Rilling. "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."