"Smaller Testicles, Bigger Parenting Role, Study Suggests"

Link here. Excerpt:

'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.
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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."
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"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.'