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. …

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.