John
Bacon, USA TODAY 4:14 p.m. EDT September 10,
2013
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."
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