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Smaller Testicles, Bigger Parenting Role, Study Suggests

Researchers explore why some dads are more nurturing

WebMD News from HealthDay

They're also stressed and tired, survey finds.

By Mary Brophy Marcus

HealthDay Reporter

MONDAY, Sept. 9 (HealthDay News) -- Are men with smaller testicles more involved dads? Could be, say the authors of a new study.

Anthropologists from Emory University in Atlanta wanted to try to better understand why some men are more actively engaged in child rearing than others, said study lead author James Rilling.

"We know children with involved fathers -- at least in modern western societies -- have better developmental outcomes socially, psychologically, and educationally. Yet, some men choose not to be involved," he said.

So the study authors decided to investigate whether anatomy or brain function explained the variation in parenting styles.

The research, published in the Sept. 9 issue of the Proceedings of the National Academy of Sciences, included 70 men who were the biological fathers of children between the ages of 1 and 2. All of the men lived with the biological mother of their child. They ranged in age from 21 to 43.

Rilling and his colleagues took blood tests at the start of the study to measure the men's testosterone levels. They also conducted interviews with the fathers and mothers separately about how involved their partner was with their child: how often did they change diapers, feed and bathe their little one, prepare a meal, take the child to the doctor?

"We relied on the mothers' reports because we thought that would be less biased," said Rilling, an associate professor of anthropology, psychiatry, and behavioral sciences.

The researchers also measured each man's brain activity using functional magnetic resonance imaging (fMRI) while the fathers viewed photos of their children with various expressions: happy, sad and neutral. A structural MRI was also used to measure the size of each man's testicles.

The findings, Rilling said, suggest that "men with smaller testes and lower testosterone levels were more involved in care-giving. The men with smaller testes volume also had a stronger neural response -- the fMRI showed more activity in the ventral tegmental area, a reward center of the brain -- when the men viewed images of their children."

The researchers concluded that while the new findings suggest there's a link between testes size and a man's involvement with his kids, anatomy isn't a sure predictor of a male's parenting potential.

"It could also be that when men become more involved as caregivers, their testes shrink," he added.

Dr. Joseph Alukal, an assistant professor in the department of obstetrics and gynecology and urology at NYU Langone Medical Center in New York City, said the researchers are addressing a complex issue, and the study makes some scientific assumptions.

"They've assumed a few things and I'm not sure they have the science to back it up," he said.

"You can't correlate testes size to hormones. Testes size -- barring an injury -- is very much stable. Testosterone level is not," Alukal said. He noted that testosterone levels are "hugely variable" depending on the time of day and other factors.

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