Researchers at Emory University are thought to have found the first correlation between testicle size and parenting, according to a new study published in the journal Proceedings of the National Academy of Sciences.
It had already been established that more testosterone meant worse fathers. As Nature magazine points out, it is also known that male chimpanzees, who do not provide paternal care, have large testes, where gorillas have small testes and are typically loving fathers.
The same was found in humans. Men with smaller testicles were more likely to be involved as a parent, performing tasks such as changing diapers, feeding, giving baths and caring for the child when he or she is ill.
To get their findings, researchers analyzed the brain activity of 70 men using an MRI scanner, and measured the size of their testicles.
The men were then given pictures of their children. Those dads with smaller testes had the part of their brains associated with caring and tenderness show stronger activity at the sight of their kids than those with large testes.
The reason for the connection between parenting and testes is still up for some debate.
"We're assuming that testes size drives how involved the fathers are," said study author James Rilling, "but it could also be that when men become more involved as caregivers, their testes shrink."
Rilling wrote: "Environmental influences can change biology. We know, for instance, that testosterone levels go down when men become involved fathers."