In 2012, MIT cancer biologist Susan Erdman and evolutionary geneticist Eric Alm set out to study whether yogurt prevents obesity. They weren't looking at balls. They weren't studying sexual health. But their mice had other plans — and what happened next produced one of the most entertaining (and legitimately important) findings in reproductive science.
The experiment started simply. Erdman and Alm were following up on a long-term Harvard School of Public Health study that had found yogurt, more than any other food, helped prevent age-related weight gain in humans. They wanted to replicate the finding in mice to understand the mechanism.
They took 80 mice — 40 males and 40 females — and split them into groups. Half ate a standard mouse diet, half ate a high-fat, low-fiber diet designed to mimic human junk food. Then they supplemented half of each group's diet with vanilla yogurt containing probiotics.
They expected to measure obesity rates and related health complications, including cancer risk. What they got instead was a masterclass in accidental science.
Researchers noticed the yogurt mice were "incredibly shiny." Using histology and cosmetic rating scales, they found these animals had 10x the active hair follicle density of other mice — luxuriantly silky fur that was unmistakable.
Then they noticed something about the males specifically: they were carrying themselves differently. Strutting. Projecting their testicles outward. The researchers could only describe it as "mouse swagger."
A lab technician noticed the testicles were visibly larger. When they measured, the yogurt mice's testicles were 5% heavier than normal-diet mice and 15% heavier than junk-food mice.
In mating experiments, yogurt-eating males inseminated partners faster and produced more offspring. Females on yogurt diets gave birth to larger litters and weaned them more successfully.
For soft tissue like testicles, weight and volume track essentially 1:1 because tissue density stays constant. So 5–15% heavier means 5–15% more volume. In terms of visual size (diameter), a 15% volume increase translates to roughly a 4.8% increase in diameter — noticeable if you're looking, but not dramatically larger to the naked eye.
What was dramatic was the behavioral change. The mice weren't just slightly bigger — they were carrying themselves completely differently. The swagger was visible to researchers before anyone even thought to measure anything.
"You know when someone's at the top of their game, how they carry themselves differently? Well, imagine that in a mouse."
— Susan Erdman, MIT cancer biologist, to ABC News
"We knew there was something different in the males, but we weren't sure what it was at first. She noticed their testicles were protruding out really far."
— Susan Erdman, on the lab technician's observation
While the "mouse swagger" headline dominated the coverage, the female results were equally striking. Female mice on the yogurt diet gave birth to larger litters and weaned their offspring more successfully than control females. Both sexes were slimmer and had visibly healthier coats.
The researchers were originally studying obesity, and the yogurt did help with that too. But the reproductive effects completely overshadowed the weight findings.
Erdman and Alm concluded that the probiotic microorganisms in the yogurt — not the yogurt itself — were driving the effects. Their hypothesis: the probiotics made the animals leaner and healthier overall, which indirectly boosted reproductive fitness and hormonal optimization.
Your gut microbiome influences far more than digestion. Gut bacteria play a role in regulating hormone production, including testosterone. Healthier gut flora can improve nutrient absorption, reduce systemic inflammation, and optimize the hormonal environment that drives reproductive development.
The yogurt mice weren't getting a "testosterone supplement" — they were getting a healthier internal ecosystem that allowed their bodies to function at a higher baseline. The bigger testicles and swagger were downstream effects of overall better health.
Here's where it gets really interesting. The MIT study was in mice — but Harvard nutritional epidemiologist Jorge Chavarro was already investigating whether similar effects existed in humans.
Chavarro and his team looked at the association between yogurt intake and semen quality in men. While the research was ongoing at the time, Chavarro told Scientific American that preliminary findings were "consistent with what they see in the mice."
To be clear: nobody has proven that eating yogurt gives human men bigger testicles. But early human data on yogurt and semen quality aligned with the mouse findings, and the gut-microbiome-to-reproductive-health pipeline is now an active and growing area of research.
What we do know in humans:
When this study came out in 2012, I was in my mid-teens. And yeah, I was already eating a lot of probiotic-rich foods — yogurt, kefir, fermented stuff. Not because I'd read any studies. I just liked it.
Did that contribute to anything? Honestly, I have no idea. The MIT study was in mice, not humans. Correlation isn't causation. I could just as easily credit genetics, timing, or pure luck. But it's a fun thing to wonder about, and the science behind the gut-hormone connection is real and growing.
What I can say for certain: your gut health matters for your overall health, your hormonal health, and — quite possibly — your reproductive health. Eating probiotic foods is a good idea regardless of whether it makes your balls bigger. That's just a potential bonus.
Let's separate the viral headline from the practical takeaway.
You can't change your genetics. But you can optimize the environment in which those genetics express themselves. A diet rich in probiotics, fermented foods, and whole nutrients — and low in ultra-processed junk — supports the hormonal environment that drives healthy reproductive development. The MIT mice proved that diet matters for reproductive fitness. The junk-food mice got the smallest balls. Don't be a junk-food mouse.
The "mice with bigger balls" headline is funny. It's also been used to dismiss the research as frivolous. That's a mistake.
The Erdman and Alm study was one of the earliest demonstrations that the gut microbiome has direct, measurable effects on reproductive organs and sexual behavior. That finding has since been confirmed and expanded by dozens of studies. The gut-reproductive axis is now a legitimate field of research with implications for fertility treatment, hormonal disorders, and sexual health.
The study was published in Scientific American (May 2012) and reported by MIT, ABC News, and multiple medical outlets. While the full paper remained unpublished at the time of media coverage, the findings were presented at scientific conferences and the lead researchers — both faculty at MIT — openly discussed methodology and results with peer review in mind.
The fact that it started with vanilla yogurt and ended with mouse swagger doesn't make the science less real. Some of the best discoveries in history were accidents. Penicillin was a moldy petri dish. Viagra was a failed blood pressure medication. And the connection between gut bacteria and reproductive health? That was a lab tech noticing some mice were walking funny.
If this article makes you want to buy a testosterone-boosting probiotic supplement from some influencer's Instagram, stop. Most "T-booster" supplements are unsupported by evidence and some contain undisclosed active ingredients that can be dangerous. If you want the benefits the MIT mice got, eat actual yogurt with live cultures, fermented vegetables, kefir, and whole foods. Skip the pills. Read our article on the $5 billion enhancement pill scam.
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Use the Penis Calculator →Also read: You Didn't Earn Your Big Dick: The Genetic Lottery Nobody Talks About — the science of why size is mostly luck, and why that should make everyone feel better.