Photo via Alex Romp/Flickr
Bigger balls may equate to more aloof fathers, or so says new research published today in the Proceedings of the National Academy of Sciences. In the study from Emory University, researchers were able to demonstrate a correlation between larger testes and a limited paternal interest in childcare.
The objective of the study was to better comprehend why some men shy away from child rearing behavior. “We know that children with involved fathers have better development outcomes,” said James K. Rilling, professor of anthropology at Emory and one of the researchers on this project. “We are interested in identifying variables that help to explain why some men get involved whereas others do not.”
An evolutionary theory called life history theory guided the researchers’ study. Life history theory seeks to explain differences in organisms' life and reproductive strategy—for example, why a mayfly lives for such a short period of time while Galapagos tortoises live for centuries. The theory suggests that resources constraints influence an organism to focus on mating or on parenting in an effort to maximize its own ability to survive and reproduce.
With this idea as a framework, Rilling and his colleagues hypothesized that there would be an inverse correlation between the size of the testes, which is very obviously related to mating, and parenting effort. They also believed that a scan of the brain would demonstrate evidence of interest in childcare or lack thereof.
The study focused on 70 biological fathers, each with a toddler between the ages of one and two, and each of whom lived with the biological mother of that child. The couples were interviewed about the man’s parental involvement. Then the men were poked and prodded to determine their testosterone levels, brain activity when presented with images of their children, and testicular volume.
What the Emory team found confirmed their original suspicions. “Collectively, these data provide the most direct support to date that the biology of human males reflects a trade-off between mating and parenting effort,” states the report. Their study showed that testicular volume was indeed inversely related to caregiving.
Furthermore, fMRIs of the fathers’ brain activity in a region called the ventral tegmental area, which is associated with reward and motivation, displayed a predictive correlation with caregiving. “The men with smaller testes were activating this brain region to a greater extent when looking at photos of their own child,” noted Jennifer S. Mascaro, another one of the researchers, in a release.
There are two major caveats to understanding these findings. The first involves the evergreen debate of nature versus nurture. “We don’t know if testes size is genetically inherited or whether it can be influenced by developmental environments,” Rilling said. “Girls raised without fathers reach menarche much earlier. It is possible that boys raised without fathers experience a biological shift towards a mating effort strategy that involves an increase in testes size.”
The second caution is that while the results do demonstrate evidence for a correlation between testes size and parenting, the researchers could not ascribe a direction of causality. Do large testes predict a lack of paternal caregiving? Or do testes shrink when men get more involved with their children? We don’t know yet.
“Our findings raise the possibility that some men may be more naturally inclined towards childcare than others,” Rilling said. “However, I don’t believe that having large testes should be used as an excuse for not becoming involved as a father.”