A Man’s Testicle Size May Influence His Enthusiasm for Parenting

The size of a man’s testicles may predict how enthusiastically he participates in parenting, according to a new study published in Proceedings of the National Academy of Sciences. Men who are more well endowed in this particular department tend to be less involved in run-of-the-mill parenting, like tucking their kids in for bedtime, feeding them or taking them to the doctor.

Rather than having an inherent interest in male anatomy, the researchers conducting the study wanted to find out why, from an evolutionary standpoint, some fathers are less invested in in rearing their children, despite the well-documented pay-offs for their gene pool. “Life History Theory offers an explanation for variation in parental investment by positing a trade-off between mating and parenting effort, which may explain some of the observed variance in human fathers’ parenting behavior,” the researchers write. In other words, some men may be naturally inclined to go down the long-term investment parenting route, whereas others may lean towards the Johnny Appleseed approach.

To test out this hypothesis, the researchers recruited 70 men who had a biological child between the ages of one and two and were living with both the toddler and his or her mother. To make sure the dads weren’t overstating their parenting prowess, the scientists interviewed mothers and fathers separately to assess how involved the men were in tasks like changing diapers, wiping their kid’s runny nose and taking on feeding duties. The Guardian elaborates:

Their answers were ranked from one to five, with one meaning the mother was almost always responsible, and five meaning the father was the main carer. The lowest male score was in the low 40s; the highest scores, from stay-at-home-dads, were in the 80s.

The researchers also took a few measurements from each of the men, including testosterone levels and testes volume. Rather than get their hands dirty, the team used magnetic resonance imaging—a more exact and less invasive method than the alternative. The average guy’s testicles measured 38 cubic centimeters, though the spectrum ranged from 18 to 60 cubic centimeters.

Finally, they hooked the guys up to a brain scanner, showed them adorable photos of their own child and observed how activity in the men’s ventral tegmental area, a part of the brain involved with chemical rewards and motivation, changed upon seeing their little ankle biter.

After analyzing the evidence, the results were clear: men with larger testicles were less-involved pops, both in their lives and in the minds. Likewise, those with higher testosterone levels were less involved parents, though levels of that hormone did not significantly influence their reaction to their child’s photo.

Of course, men are not monkeys or marmots. More goes into their behavior and parenting decision than pure instinct alone. The researchers point out that their results are only correlational, meaning testicle size does not cause men to be good or bad fathers. ”We are not saying you can determine a man’s parenting aptitude based on their individual biology,” the researchers summarized for the Guardian. ”But it does suggest that some men may be wired to participate in childcare more than others.”

More from Smithsonian.com:

Pfizer’s Recipe for Pig Testicle Tacos 
Fathers Recognize Their Babies’ Cries Just as Well as Mothers 

Rachel Nuwer | | READ MORE

Rachel Nuwer is a freelance science writer based in Brooklyn.