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.

While the Emory researchers took a single blood test to measure testosterone levels in the men, Alukal suggested that it might have been more accurate to take multiple blood tests at the same time of day over a longer period of time and use an average.

He said the fMRI findings raise questions, too.

"What does the fMRI tell us in this regard? I don't know. fMRI has helped us with understanding brain activity, but there's a totally different interaction going on between a parent and a child compared to just being shown a picture of one's child. It's a poor stand-in," said Alukal.

He added: "The study scratches at the surface of the complexity of this subject."

More information

See ChildCare Aware of America for tips on becoming a better dad.