Contrary to popular opinion, bigger may not always be better.

New research from Emory University has found that fathers with smaller testicles are more likely to be involved in the care and nurturing of their children, Medical Daily reported.

According to lead author Dr. Jennifer Mascaro, the goal of the study was to find a biological mechanism that could potentially explain why some fathers devote more time to the parenting of their children than others.

“Our study is the first to investigate whether human anatomy and brain function explain this variance in parenting effort,” Mascaro, an anthropologist and neuroscientist at Emory, said.

In a new study published in the Proceedings of the National Academy of Sciences, Mascaro and her team recruited 70 fathers with newborns between the ages of 1 and 2 years old. Both the dads and their children’s biological mothers were asked to complete surveys regarding the fathers’ involvement in hands-on childcare. The study did not receive federal funding.

The researchers then tested the men’s testosterone levels, as lower levels of the sex hormone have been associated with greater parental involvement, according to Medical Daily. The fathers also received MRI scans of their brains to measure activity in the ventral tegmental area (VTA), an area of the brain that governs reward and parental motivation. MRI scans were also done on the men’s testes to measure volume.

The fathers with smaller testicular volume had more brain activity in their VTAs and scored higher on the parenting surveys. Additionally, those with less testosterone had a modest association with better caregiving, a finding the researchers linked with testicle size, since testosterone is produced in the testes.

Though the results may seem concerning, the researchers emphasized that testicle size does not necessarily control a father’s parenting skills.

"We're assuming that testes size drives how involved the fathers are," said Emory anthropologist Dr. James Rilling, whose lab led the study, "but it could also be that when men become more involved as caregivers, their testes shrink. Environmental influences can change biology. We know, for instance, that testosterone levels go down when men become involved fathers."

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