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What a man has in his pants predicts his parenting skills, a new study suggests.

Not only do men with smaller testicles make better fathers, they also have brains that show more nurturing-related activity when looking at pictures of their own children. 

Researchers at Emory University found that men with smaller testicles are significantly more likely to be involved in hands-on care of their toddlers.

Scientists wanted to understand why some fathers invest more energy in parenting than others. While economic, social and cultural factors are obvious factors, researchers wanted to see if there is a deeper, biological cause.

"It's an important question because previous studies have shown that children with more involved fathers have better social, psychological and educational outcomes," researcher James Rilling, an anthropologist at Emory University, said in a news release.

Researchers said the latest study is the first to look at whether human anatomy and brain function explain differences in parenting effort.

Previous studies revealed that lower levels of testosterone in men have been correlated with greater paternal involvement, and that higher levels of the hormone predict divorce as well as polygamy.

The latest study involved 70 biological fathers who had a child between the ages of one and two, and who were living with the child and its biological mother. Researchers interviewed both the mother and father about the father's involvement in hands-on childcare, including tasks such as changing diapers, feeding and bathing a child, staying home to care for a sick child or taking the child to doctor visits.

Researchers measured the men's testosterone levels, brain activity and testicles size. Researchers scanned the men's brains as they viewed photographs of their own child with happy, sad and neutral expressions, and similar photos of an unknown child and an unknown adult. Afterwards the participants' testicular volume was determined using a structural MRI.

The findings revealed that both testosterone levels and testicle size were inversely correlated with the amount of direct paternal care giving reported by the parents.

Researchers also found that testicular volume correlated with activity in the ventral tegmental area (VTA), a part of the brain system associated with reward and parental motivation.

"The men with smaller testes were activating this brain region to a greater extent when looking at photos of their own child," explained lead researcher Jennifer Mascaro.

Researchers theorize that testosterone levels may be more related to pre-copulatory and intrasexual competition, whereas testicular volume may reflect post-copulatory mating investment.

However, researchers do not know if smaller testes lead to better father or it being a more involved father shrinks a man's testicles.

"We're assuming that testes size drives how involved the fathers are," Rilling explained. "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."

While the correlation between testes size and care giving is statistically significant, researchers noted that there was some variance.

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

"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 care giving activities, but that by no means excuses them," he concluded.

The findings are published in the journal Proceedings of the National Academy of Sciences.