Men with larger testicles tend to be less involved fathers than those with smaller testes, a new study suggests.
The findings, detailed today (Sept. 9) in the journal Proceedings of the National Academy of Sciences, are correlational, so they can't say exactly why the trend exists but only that there is a link.
But men who produce more sperm have bigger testes, and sperm production is extremely energy intensive for the body, so it may be that fathers "face a trade-off between investing energy in parenting and investing energy in mating effort," said study co-author James Rilling, an anthropologist at Emory University in Atlanta. [Sexy Swimmers: 7 Surprising Facts About Sperm]
Scores of studies have shown that children with involved and caring fathers tend to do better emotionally, socially and educationally.
So Rilling and his colleagues were interested in understanding what makes some men stellar dads and others AWOL.
A 2011 study in the Philippines suggested that men who have high testosterone levels are more likely to marry. Even so, those men who are eventually more involved in day-to-day child care duties - such as changing diapers, running the bath or kissing scraped boo-boos - see their testosterone levels drop more than men who remain aloof after having children.
But testosterone has many roles in the male body, so it wasn't clear whether the drop in the male hormone occurred because men were investing more in parenting than in mating.
Rilling and his colleagues surveyed 70 married men ages 21 to 55 who had between one and four children about their involvement in caregiving. Only four of the men routinely did more caregiving than the mothers.
The researchers then used magnetic resonance imaging to scan the brains of the fathers while they looked at images of their children. The team also scanned the participants' testes to assess volume and measured testosterone levels from blood samples.
Men's testes' volume varied considerably - from a little more than a tablespoon in volume to about a quarter cup.
Men with bigger testes had a more hands-off parenting style, and the reward centers of their brains activated less when the men viewed their children's pictures. These fathers also tended to have higher testosterone levels.
The findings are fascinating and provocative, said Sarah Hrdy, an anthropologist at Citrona Farms who was not involved in the study.