Men with smaller testicles tend to be more involved as fathers, a new study suggests.

James Rilling and his colleagues at Emory University in Atlanta, Georgia, used MRI scans to measure the volume of the testicles of 55 fathers. The team asked these men – and the mothers of their children – a series of questions to determine how involved they are as fathers. The team also used fMRI brain scanners to look at the brain activity of the men as they viewed photos of their kids.

Men with smaller testicles got the best parenting scores in the questionnaires. When looking at pictures of their children, these men also showed more activity in regions of the brain associated with empathy and motivation to care for offspring than men with bigger testes, which suggests they are more nurturing fathers.

Previous studies have suggested that men with more testosterone are more sexually active, but that levels of the hormone drop once men become fathers. But the new link between testicle size and involvement in parenting could not be explained by the participants’ testosterone levels.


“Circulating testosterone isn’t the whole story,” says Susanne Schultz, an evolutionary biologist at the University of Manchester, UK, who describes the finding as “a very surprising result”.

Energy equation

Men with bigger testicles tend to produce more and higher-quality sperm. There may be a trade-off between the amount of energy a man invests in making sperm, which can be thought of as a mating strategy, and parenting, Rilling says. “We all do really have a finite amount of energy that we can spend on things,” he says.

Rilling does not know how smaller testicles might make men better fathers. “We’re assuming that testis size drives how involved the fathers are, but it could be that when men become more involved as caregivers, their testes shrink.”

The findings do not necessarily mean that men with bigger testicles will be worse fathers. Most fathers choose how involved they are in their child’s upbringing, says Rilling. “It might be more challenging for some men to do these kinds of caregiving activities, but that by no means excuses them,” he says.

Journal reference: PNAS, DOI: 10.1073/pnas.1305579110