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Men with smaller testicles are more likely than their well-endowed brethren to be involved in the care of their toddlers, anthropologists at Emory University report.
The higher the testosterone levels and larger the testicles, the smaller the amount of direct paternal caregiving by dads 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.
The goal of the research, Rilling says, was 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.
Rilling noted that "life history theory" holds that evolution optimizes use of resources toward mating or parenting to generate the largest number and healthiest offspring.
The report notes that economic, social and cultural factors could also influence a father's level of caregiving. Although statistically significant, the correlation between testicle size and caregiving 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.
Magnetic resonance imaging also was used to measure brain activity and "testicular volume."
"We're assuming that testes size drives how involved the fathers are," Rilling says, "But it could also be that when men become more involved as caregivers, their testes shrink. Environmental influences can change biology."
Some researchers question the study's findings.
Lee Gettler, an anthropologist at Notre Dame who has conducted research on how men respond physically to father-child interactions, says the study assumes that larger testicles translate to more "mating" by men. But they don't, Gettler told CNN. "Large testes do not make you act promiscuously or badly as a parent."
Abass Alavi, a researcher with the University of Pennsylvania, told CNN that size doesn't even determine how much sperm is being created.
"What is important is how much sperm the testicle is making," he said. "Some geniuses have very small brains."