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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, isn’t 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 that’s 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 they’ll 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.

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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 men’s level of parenting involvement. (To eliminate positive bias, they used the women’s 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.

The MRI scanned the brains of the men while they looked at pictures of a strange adult, a strange child, and their own toddler all expressing “sad,” “happy” and "neutral" emotions. Then the MRI scanned the men’s testicles to check the volume.

Men who more strongly engaged the brain’s pleasure centers, specifically the ventral tegmental area (VTA), in response to images of their own babies were also more likely to be nurturing according to the surveys. Men with strong VTA activation also tended to have smaller testicle volume.

Testosterone levels were not related to VTA activity and there was a weak correlation between testes volume and testosterone levels.

“The correlation is stronger between sperm count and sperm quality and testicle size,” Rilling explained in an interview. “Bigger size, better, healthier sperm.”

Larger size would seem to indicate men built for competition with other men to make women pregnant, but not so much to care for the child.

Gettler praised the study for its pioneering use of both physiological markers and brain imaging, but was skeptical testicle size really matters.

“To make their evolutionary scenario work, larger testes need to enhance fertilization likelihood so much that promiscuity (and low parenting) and large testes would co-evolve and become genetically correlated,” Gettler said. “But what’s the [evolutionary] disadvantage of large testes to invested fathers?”

Rilling readily admits that the study poses as many questions as it answers. For example, which direction does the size difference flow? Do men genetically predisposed to smaller testicles become more nurturing fathers, or does nurturing shrink testicles? Does life experience alter testicle size and then affect parenting? Are some men genetically predisposed to get more brain reward from nurturing, or do men who make a conscious effort to nurture get rewarded and learn to like it? Or both?

These questions are more than curiosities to Rilling.

"It’s important to think about possible interventions for men who are naturally less inclined to becoming involved parents,” he said. “Here we are studying normal variation. But some men have post-partum depression. Some lack all motivation. Some are abusive.”

His lab is now planning a study to see if intranasal oxytocin – a key brain hormone fostering nurturing between mothers and babies -- given to new fathers can augment brain reward in response to pictures of their own children. If so, “it might suggest possible pharmacological treatment for men who lack paternal motivation,” Rilling said.

Brian Alexander is a frequent contributor to NBC News and a co-author of “The Chemistry Between Us: Love, Sex, and the Science of Attraction.”