Wouldn’t it be awesome if there were a simple test that could determine whether your man was going to be an attentive parent or a drop-out dad? Well, a quick look down below at the size of his family jewels might be all you need, according to new research published in the Proceedings of the National Academy of Sciences and reported by Time.

After a team of researchers compared men’s testicle size with their eventual parenting ability, scientists found that the smaller a man’s testicles (a.k.a. gonads), the more active he seemed to be in his children’s lives.

To test this theory, anthropologists studied 70 biological fathers in Atlanta whose brains and balls were measured with an MRI while they looked at photographs of their children. MRI charts showed that the brain region associated with nurturing lit up most intensely in men with smaller gonads.

Scientists then asked the participants’ partners to fill out questionnaires about how involved their men were in child care tasks, such as taking their young one to the doctor or waking up late at night to calm a distressed baby. Men who got the best reviews were those with the smallest testicles.

Interestingly, earlier research suggested that men with higher levels of the male hormone testosterone are less interested in raising children. The current findings found a correlation that showed men with larger testicles who produced more amounts of semen were also less apt to exhibit parental involvement.

For families to produce well-developed, well-adjusted kids, having a dad who actively participates in child rearing is often key. “We know in modern Western societies children with more involved fathers have better developmental outcomes,” said James Rilling, PhD, a professor of anthropology at Emory University and coauthor of the study.

Families with absent fathers are more likely to be African American. But fatherhood programs in black communities are showing men how to become better dads. Click here for more information.