A study looking at penis sizes from 1942 until 2021 has found that the average erect penis length has increased significantly in the last 30 years, while flaccid penis lengths remained the same. While this may sound like good, or at least neutral, news, the lead researcher says that it could be caused by factors such as chemical exposure interacting with our hormones.

The meta-analysis looked at penis length studies from the last eight decades, including data from 55,761 males all over the world. Studies were only included if the measurement was carried out by an investigator, excluding any that relied on self-reported data. They also only included studies that measured penises "from the root (pubo-penile junction) of the penis to the tip of the glans (meatus) on the dorsal surface".

While no increase or decrease was found in flaccid or stretched penis length, the team found a significant increase in erect length.

"Erect length increased significantly over time in several regions of the world and across all age groups," the team wrote in the study. "After adjusting for geographic region, subject age, and subject population; erect penile length increased 24 percent over the past 29 years".

That's an increase from 12.3 centimeters (4.8 inches) to 15.2 centimeters (6 inches) in just under three decades. 

The authors explained that they had been motivated to compare penis sizes over time because of other changes to men's reproductive health in recent years, such as declining sperm count.

"Given the trends we'd seen in other measures of men's reproductive health, we thought there could be a decline in penile length due to the same environmental exposures," lead author Michael Eisenberg, professor of urology at Stanford Medicine, said in a press release.

That wasn't the case, but Eisenberg believes that the increase seen could be the result of changes to environments.

"There could be a number of factors at play, such as chemical exposure, like pesticides or hygiene products, interacting with our hormonal systems. These endocrine-disrupting chemicals – there are many – exist in our environment and our diet," he said. "As we change our body's constitution that also affects our hormonal milieu. Chemical exposure has also been posited as a cause for boys and girls going into puberty earlier, which can affect genital development."

The team noted that other factors, such as techniques for measuring length, temperature, and arousal state of the participant, could affect the results. However, they believe that it warrants further study, given the implications it could have for male reproductive health.

"The increase happened over a relatively short period of time," Eisenberg added. "If we're seeing this fast of a change, it means that something powerful is happening to our bodies. We should try to confirm these findings and if confirmed, we must determine the cause of these changes."

The study was published in the World Journal of Men's Health.