Perhaps it’s time to stop obsessing over penis size, and start to think more about those underloved lads underneath. A new study has suggested that testicle size plays a role in whether or not a guy is an involved dad, but this is one time less is more: the smaller the family jewels, the better the family man.

Prior research has already suggested that dudes with higher testosterone levels are less into raising kids, but this study, which was published in the Proceedings of the National Academy of Sciences on Monday, is the first to find an independent correlation between testicle volume and parenting. As with other seed-bearing nuts, testicle size determines how much juice is produced, and it seems there’s a kind of law of diminishing returns at work. The greater the semen output in each ejaculation, the smaller the parenting output later on. That matters — a lot.

“We know in modern Western societies children with more involved fathers have better developmental outcomes,” says one of the study’s authors, James Rilling, associate professor of anthropology at Emory University in Atlanta. Rilling and his co-authors wanted to see if they could figure out why some fathers opt out of their kids’ lives — the number of absent fathers, the study notes, increased “precipitously” in the latter half of the 20th century. Was there some biology involved? Testosterone is one factor, but it can vary widely in a guy because of age or diet or general health. “Testicular volume is likely to be a more stable measure than testosterone,” says Rilling.

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To test their theory, the anthropologists found 70 biological fathers in Atlanta who were happy to have their brains and their berries measured by an MRI. The brains were monitored while the men looked at photographs of their child. The MRIs showed that the region of the brain most associated with nurturing lit up more robustly in guys with smaller gonads. The study also noted that these men’s brains responded particularly fiercely to pictures of their children displaying some emotion.

In addition, the researchers had the men’s partners fill out questions about how involved their men were in child-care tasks such as taking kids to the doctor and getting up at night to respond to them if they wakened. And true to crude jokes around this issue, fathers who got the thumbs-up from their partners were those with the smaller cojones.

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Before women start weighing up potential life partners by what they feel when the men look straight ahead and cough, however, it should be noted that it’s not clear what comes first. Does simply having kids shrink the size of testicles? Unlikely, says Rilling. Nonfathers’ scrota were also put under the MRI, to check that parenting was not the sole determinant of size. “We don’t know the direction of the causality,” he admits. “It could be that as men become more involved in caregiving the testes shrink.” But he believes it’s more likely that guys with a little less in the sack are a little better with the crib.

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There’s a branch of evolutionary psychology known as life-history theory, which suggests that since a human’s energy is limited, there is always a competition for resources between mating and parenting. Consider the steep decline in sex that parents of young children often experience because of sheer exhaustion. Life-history theory suggests a similar type of equation happens on a specieswide scale: energy either goes to raising the young or making more young ones. “Collectively, these data provide the most direct support to date that the biology of human males reflects a trade-off between mating and parenting effort,” claims the study.

While this is unlikely to herald a change in the way women seek mates, we may be at the beginning of an era when being called ballsy is no longer just a compliment.

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