The happy father in the photo above likely has tiny little itty bitty testicles. His lil’ baby balls are probably so small he can hardly see them when he showers. How do we know? Because he looks empathetic, nurturing, and involved in his daughter’s life–all qualities that decrease when a man has great big cojones, according to a new study.

New Scientist explains:

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.

But why? According to the researchers, it may have to do with sperm. Big-balled men are wont to produce strong, fertile sperm that’s likely to make a baby, meaning there may not be much evolutionary reason to be a good dad. In other words, if you’re a guy with tiny genitals, it’s more in your interest to make sure the kids you do have turn out okay.

Frighteningly, there’s also the possibility that the cause-effect relationship is flipped. “We’re assuming that testis size drives how involved the fathers are,” wrote James Rilling of Emory University, “But it could be that when men become more involved as caregivers, their testes shrink.”