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Study: Smaller the Testes, Better the Dad?

A new study claims smaller testes mean dads are more involved with their kids.
Photograph by Getty Images/iStockphoto

If it's been a while since you told a good "size matters" joke, get ready for some new material...

Anthropologists recently set out to see just why some dads are more hands-on than others, and the research brought them to one interesting conclusion: It may have something to do with the size of their packages. (Yes, seriously.)

Go ahead and laugh, because we certainly did. But then hear it out: The study followed 70 men in total, all of whom were the biological fathers of kids ranging from 1 to 2 years old, living at home with the mother of their child, and ranging in age from 21 to 43. Researchers first took blood tests to scope out the men's testosterone levels. Then they conducted interviews—first with the dads, and then with the mothers of the children—about just how involved the men actually were with their children. Did they willingly step up for diaper duty? Did they take charge during meals and bath time? How often did they take their kid to the doctor?

“We relied on the mothers’ reports because we thought that would be less biased,” said study researcher James Rilling, an associate professor of anthropology, psychiatry and behavioral sciences. (Good thinking there, Rilling.)

The study's leaders also gave each man an MRI to measure their brain activity while photos of their kids flashed before their eyes—all showing various moods like happy, sad or just so-so. And last, but not least, a structural MRI measured the size of each dad's testicles.


As Rilling told HealthDay News, the up-close and personal look at each dad's manly bits clearly showed them that "men with smaller testes and lower testosterone levels were more involved in caregiving. The men with smaller testes volume also had a stronger neural response–the MRI showed more activity in the ventural tegmental area, a reward center of the brain—when the men viewed images of their children.”

But before you go thinking that a man's pre-kid sac size is a clear indicator of whether or not he'll be a great dad, hold up for a minute. Rilling pointed out that the size correlation could actually hint at the opposite: Dad's testicles could actually be shrinking the more involved he becomes as a caregiver. (...If that's any consolation?)

Still, not everyone's buying what Rilling is selling. Dr. Joseph Alukal, an assistant professor in the department of obstetrics and gynecology and urology at NYU Langone Medical Center in New York City, said that a few unfounded conclusions have been made by the researchers.

“You can’t correlate testes size to hormones. Testes size—barring an injury—is very much stable. Testosterone level is not,” Alukal said. He noted that testosterone levels are “hugely variable” depending on the time of day and other factors. (Hmmm ... we may not be scientists, but just anecdotally, he does have a point there.)

Alukal also noted that the researchers probably should have taken not just one, but many blood samplings from the men at the same time of day, over a longer period of time for truly accurate results.

And as for those MRIs?

“What does the MRI tell us in this regard?" Alukal said. "I don’t know. MRI has helped us with understanding brain activity, but there’s a totally different interaction going on between a parent and a child compared to just being shown a picture of one’s child. It’s a poor stand-in,” said Alukal.

OK, so maybe we can't totally hang our hat on this study yet, as Alukal notes that it merely "scratches at the surface" of a complex subject. But it certainly looks like they're on to something interesting...

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