Men with larger testicles tend to be less involved fathers than those with smaller testes, a new study suggests.
The findings, detailed in the 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.
Scores of studies have shown that children with involved and caring fathers tend to do better emotionally, socially and educationally.
A 2011 study in the Philippines had already 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.
Those surveyed were 70 married men ages 21 to 55 who had between one and four children. Only four of the men routinely did more caregiving than the mothers. The men with bigger testes had a more hands-off parenting style, and as fathers, they tended to have higher testosterone levels.