Big testicles indicate rubbish dads, study says

It may sound like a load of balls, but scientists have claimed that the size of a man’s testicles will determine how active he will be in bringing up his kids.

Experts said those with smaller testicles were more likely to be hands-on dads and play their part in daily chores such as nappy- changing, feeding, and bathing their children.

However, men with larger pairs are more interested in procreating than raising kids, according to the study.

Researchers in Emory University in Atlanta were probing an evolutionary theory about trade-offs between investing time and effort in mating or putting that energy into bringing up youngsters.

The study, published in Proceedings of the National Academy of Science, examined the relationship between testicle size and fatherhood in 70 men who had children between the ages of one and two.

The research involved the men undergoing brain scans while being shown pictures of their tots.

Scientists discovered that those with smaller testicles tended to have a greater response in the reward area of the brain than those with a larger size.

MRI scans showed a three-fold difference between the volumes of the smallest and largest testicles in the group.

Those at the smaller end of the study were also more likely — as interviews with the man and the mother proved — to be more active in parenting duties.

Researcher James Rilling told the BBC: “It tells us some men are more naturally inclined to care- giving than others, but I don’t think that excuses other men. It just might require more effort for some than others.”

The scientists said they believe the size of the testicles, probably through the hormone testosterone, affects behaviour.

Dr Rilling added: “We know, for instance, that testosterone levels go down when men become involved fathers.”

However, the research team conceded that further tests analysing the testicle size before and after becoming a father are needed to cement their theory.

Cultural and societal expectations on the role of the father were not accounted for in the study.

All of the men in the study were from the Atlanta area, meaning the relative impact of society and biology was not measured.

Levels of promiscuity and testicle size are strongly linked in animals — with those with the largest pair tending to mate with more partners.