Men with smaller testes are more likely to be involved in the care of their toddlers than their well-endowed brethren anthropologists at EmoryUniversity report.

The higher the testosterone levels and larger the testicles, the smaller the amount of direct paternal care giving by fathers as reported by parents in the study.

"Our data suggest that the biology of human males reflects a trade-off between mating and parenting," Emory anthropologist James Rilling, whose lab conducted the research, reports on the Atlanta university's website. The Proceedings of the National Academy of Sciences published details of the study this week.

Rilling research aimed to determine why some fathers work harder at parenting than others. "Previous studies have shown that children with more involved fathers have better social, psychological and educational outcomes," he told the school's website.

The report notes that economic, social and cultural factors could also influence a father's level of care giving. Although statistically significant, the correlation between testicle size and care giving was not perfect.

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

The study included 70 biological fathers who were living with their toddler and its biological mother. The mothers and fathers were interviewed separately about the father's involvement in tasks such as changing diapers, feeding and bathing a child and caring for a sick child.