Researchers Don't Know Why Average Penis Size Has Grown

With sperm counts and testosterone levels falling, researchers thought they would find that average penis sizes were shrinking as well. They were surprised to find the opposite was true. According to a study published in the World Journal of Men's Health, the average length of an erect penis has increased 24% over the past 29 years, from 4.8 inches to 6 inches. The researchers, led by Stanford University urologist Dr. Michael Eisenberg, looked at data from 75 studies from between 1942 and 2021 involving almost 56,000 men. The trend was found in "several regions of the world and across all age groups," researchers said.

"The million-dollar question is why this would occur," Eisenberg says, per USA Today. He says one possible explanation is that boys, like girls, have been reaching puberty earlier, giving all their body parts more time to grow. Dr. James Hotaling, a urologist not involved with the study, tells USA Today that another possibility is that penises haven't grown longer, but the method of measurement has changed in recent decades.

Eisenberg tells Stanford's Scope blog that further research should look into whether environmental factors like chemical exposures could be behind the change. "The increase happened over a relatively short period of time. Any overall change in development is concerning, because our reproductive system is one of the most important pieces of human biology," he says. "If we're seeing this fast of a change, it means that something powerful is happening to our bodies." He adds: "Just as we measure height and weight every year across the US, this is something else we could measure in a systematic fashion, because it may turn out to be an early indicator of changes in human development." (More penis size stories.)