Smaller testicles, more nurturing the men are with their children, according to a new study.

Emory University researchers are reporting that men with smaller testicles tend to be more nurturing fathers. This means taking care of the baby when the baby is sick. Change a diaper when a necessary or put the baby to sleep when it's bedtime.

The smaller testicles, more nurturing theory came about after the 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, which the results were published in Proceedings of the National Academy of Sciences.

"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.

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.

"We're interested in trying to identify variables why some men become more involved in caring for their children than others," Rilling said. "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."

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, according to CBS News.

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, according to the study.

Lower the testosterone, the more nurturing the fathers were with their children. With the brain scans, it showed a structure associated with reward and parental motivation, which is the ventral tegmental area. It showed more activity in dads with smaller testicles when looking at pictures of their own kids than in dads with larger testicles

"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.