Father with smaller testicles are generally more involved than their bigger-balled brother in caring for their children and respond more when looking at pictures of those children. At least that’s what researchers at Emory University in Atlanta discovered.

The research project with the brainchild of James Rolling, an anthropologist who was trying to figure out what makes some dads more involved than others. So he and his team did MRIs of the brain and balls of 70 men, all of whom had children 1-2 years old. They compared their results with surveys—filled out by the dads and the moms—about the dad’s level of involvement. And they also measured the men’s testosterone levels, finding that dads who provided more childcare tended to have lower levels.

Testicle size and sperm count are closely linked: the bigger the balls, the more sperm there is. So this team of anthropologist speculated that having more sperm would make a man want to spread it around as far and wide as he could. That would leave less time for—and interest in—child care.

The connection between the round parts of a man’s package and his level of involvement with his kids is pretty well-known—at least among other primates. Research has found that male chimps, which don’t do much to care for their offspring, have testes that are twice as large as a human male’s. But gorillas, which are very protective of their babies, have smaller balls.