Men with smaller testicles make more caring fathers, study reveals

Men's aptitude for childcare may be reflected in the size of their testes, according to a study by US scientists.

Researchers found that men with smaller testes were more likely to take charge of children's bath-time, visits to the doctor, night-time comforting, and other parenting jobs than others who have larger testicles.

The same story was borne out by brain scans which showed that men with smaller gonads reacted more strongly to photographs of their own children than did men with larger ones.

The findings are the strongest evidence yet that variations in male anatomy reflect competing evolutionary strategies that can be distilled down to mating as much as possible versus investing more in parenting. Both are effective ways to maximise an animal's chances of continuing their lineage.

Jennifer Mascaro, an anthropologist at Emory University in Atlanta, said that while her work revealed a correlation between testes size and parenting, it said nothing about the causes. She suspects the size of a man's testes influences how involved he gets in childcare. But the reverse could be equally true: indulging in childcare may make a man's testes shrink.

"We are not saying you can determine a man's parenting aptitude based on their individual biology. But it does suggest that some men may be wired to participate in childcare more than others. They may take to it more readily," Mascaro said.

Mascaro based her experiments on what evolutionary biologists call life history theory. The theory can explain how evolution shapes organisms to optimise their chances of having fertile young. The different strategies are clear in nature. Sea turtles have scores of offspring but do little to raise them, while chimpanzees have far fewer young, but invest heavily in their upbringing.