In a study published in Proceedings of the National Academy of Science, anthropologists at Emory University reported their research into why some fathers are more involved in child care than others.
"Our data suggest that the biology of human males reflects a trade-off between investments in mating versus parenting effort," Emory researcher James Rilling, said in a statement.
The finding may reflect an evolutionary trade-off between putting greater resources towards being a father or towards mating, he said.
Promiscuous primates who must compete with other males to mate tend to have larger testicles than monogamous primates in pair-bonded relationships, he said.
However, he cautioned, the findings may not be as straightforward as suggesting men with smaller testicles are destined to become better fathers, noting that economic, social and cultural factors could also influence a father's level of caregiving.
"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," Rilling said. "Environmental influences can change biology."
The correlation suggested by the study between testicle size and caregiving was not perfect, he said.
"The fact that we found this variance suggests personal choice," he said.