Apparently, if your man has big balls, he'll probably spend less time caring for your kids. Not only that, they have a decreased brain response to images of their child.

Researchers led by James Rilling of Emory University wanted to know why some fathers were more involved than others.

"Our study is the first to investigate whether human anatomy and brain function explain this variance in parenting effort," study researcher Jennifer Mascaro, said in a press release.

So they measured the testicle size of 70 new dads from Atlanta between 21 and 43 years old. They also analyzed their brain activity in regions implicated in parental motivation when shown images of their child and quizzed their wives about their man's involvement in child raising.

The size of testes is associated with sperm production and testosterone levels, which means if you have bigger balls, you produce more sperm and testosterone. Researchers found that testes volume was related to how involved a man is in his child's life.

They found that smaller balls and lower testosterone levels were independently correlated to more parental involvement, while larger testicle volume and higher testosterone levels were correlated to less involvement.

Study results are shown below - where caregiving is reported on a scale in which 72, the flat dotted line, means both parents share the burden equally. Scores below 72 mean mothers take a greater burden of the work:

We don't know what environmental factors determine testes size, or if ball size is the cause or result of different approaches to mating and parenting in males. The author suggests that more invested fathers may have been impacted by early-life stress or unpredictability, and possibly even the absence of their own fathers.

"The fact that we found this variance suggests personal choice," Rilling said. "Even though some men may be built differently, perhaps they are willing themselves to be more hands-on fathers. It might be more challenging for some men to do these kinds of caregiving activities, but that by no means excuses them."

Great. Now when we look at our man's balls we're going to assume what kind of dad they'll be. So, best go for someone with smaller balls? Does this mean Brad Pitt has really tiny balls?! Now this is news.