Aw, nuts! Nurturing dads have smaller testicles, study shows

Sep. 9, 2013 at 3:20 PM ET

Do men with small balls make good fathers? That may sound ridiculous, but Emory University scientists have found that men who tend to enjoy being a nurturing parent also tend to have smaller testicles.

The study, released today in the Proceedings of the National Academy of Sciences, and involving 70 men aged 21 to 55 with at least one child under 2 years old, isnt a joke.

Over the past decade, science has found that men across cultures undergo a transformation if they become nurturing fathers. Attentive fathers in the Philippines, Africa, Europe and North America all show significant drops in testosterone levels.

The general idea is that lower testosterone on a day-to-day basis helps attune fathers to the needs of their children, University of Notre Dame anthropologist Lee Gettler, who studies this effect, told NBCNews.

Lower testosterone may also make men more empathetic, less aggressive, less interested in mating, or all these.

The idea is part of Life History Theory. The theory holds that many animals, including people, trade off between putting resources into mating, versus parenting. The more energy devoted to having sex, and engaging in competition with others to do so, the less thats left for raising offspring, and vice versa.

The life histories of children have shown that the more stress and family disruption they experience, the greater the risk theyll face troubles later. Girls with an absent father, for example, are more likely to start their periods sooner, have sex sooner, and to become single mothers. Boys with absent fathers or stressful childhoods are more likely to begin having sex earlier. They are more likely to put more effort into mating, less into parenting.

The Emory group, led by post-doctoral fellow Jennifer Mascaro in the lab of James Rilling, is the first to use testicle size as a physical marker, and to see if testicle size correlates with brain reward positive feelings -- from nurturing as a way to help explain variation in male parenting.

They had the men and their female partners fill out a lengthy survey about the mens level of parenting involvement. (To eliminate positive bias, they used the womens answers.) Next, they tested the amount of circulating testosterone in the blood of 66 of the men. Then they scanned the men in an MRI machine.

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Aw, nuts! Nurturing dads have smaller testicles, study shows

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