Their new study shows that men with smaller testicles tend to be more nurturing fathers, more willing to change a diaper than their counterparts toting larger testes.
"Our data suggest that the biology of human males reflects a trade-off between investments in mating versus parenting effort," James Rilling, an associate professor of anthropology at Emory, said in a statement.
For the study, published Sept. 9 in Proceedings of the National Academy of Sciences, researchers recruited 70 fathers who had a biological child between ages 1 and 2, and who lived in a home with the child and its biological mother.
Matching up biological families was crucial because the researchers were testing the "Life History Theory" of evolution, which says that males only have a finite amount of resources to allocate towards reproduction. Therefore, differences in fathers' parenting skills may reflect an evolutionary trade-off between putting greater resources towards being a father or towards mating. For example, promiscuous primates that have to compete with other males to spread their seed tend to have larger testicles than monogamous primates in pair-bonded relationships, Rilling explained to CBSNews.com.
Are these sorts of trade-offs also occurring in people?
"We're interested in trying to identify variables why some men become more involved in caring for their children than others," said Rilling.
Fathers and mothers were interviewed separately and asked about dad's involvement in hands-on care tasks like changing diapers, feeding, bathing, taking the child to the doctor or staying home with him or her for sick days.
Dads also got their levels of testosterone -- a male sex hormone -- measured, before undergoing fMRI brain scans to measure activity when they looked at pictures of their own kids with happy, sad and neutral expressions. Dads also looked at similar photos of an unknown adult with a child for comparison.
The anthropologists found the smaller the size of the dads' testicles, or testes, the more caregiving was reported by both parents. Men with lower testosterone levels were also more likely to be more involved fathers.
The brain scans showed a structure associated with reward and parental motivation, the ventral tegmental area, showed more activity in dads with smaller testicles when looking at pictures of their own kids than in dads with larger testicles. Rilling, who is also a neuroscientist, said this finding suggests an underlying neurological basis for this phenomenon.
But, the findings may not be as simple as saying men with smaller testicles are destined to become better fathers, he said. The study did not prove a cause-and-effect relationship.
"We're assuming that testes size drives how involved the fathers are, but it could also be that when men become more involved as caregivers, their testes shrink," he said in the statement. "Environmental influences can change biology."
Rilling added that the link was not clear-cut across all dads studied, with some men with larger testes shown to be caring dads. That suggests some men who are "built differently" may be willing themselves to be more hands-on dads, he said, and may have to try a little harder than men who are more evolutionarily equipped.
"Some men may be more naturally inclined towards getting involved in child-rearing than others," he said. "But I don't think that excuses any men with large testes from getting involved...It's really important for all fathers to make an effort to become involved," added Rilling.
Dr. Joseph Alukal, an assistant professor of urology at NYU Langone Medical Center in New York City, who was not involved in the study, told HealthDay that, "the study scratches at the surface of the complexity of this subject," but he questioned the findings, noting there's a lot more to parenting and brain activity that can be measured from looking at a picture of one's own child.
"There are lots of other variables that affect fatherhood," added Dr. Charles Snowdon, a professor of psychology and zoology at the University of Wisconsin-Madison, to Nature. For example, some men may be more involved fathers because they used to look after younger siblings, he said.
While Rilling concedes these are limitations of his study, he says that there are other ways to care for a child, like through financial support or mentoring. The bottom line? Men who want to be good dads are capable of becoming more involved, he emphasized.
"We have big brains that can override some of these large predispositions," he said. "I think all men are capable of becoming involved fathers if they really want to."