Penis lengths may not be shrinking — new evidence suggests they’re growing at an alarming rate, and scientists don’t know why

They found that overall the average erection length has increased from 4.8 inches to 6 inches in the last three decades. 

The finding was a surprise to researchers, who expected to find a decline in penis length, coinciding with other recent trends in men’s reproductive health such as lower sperm counts and testosterone levels, according to senior study author Dr. Michael Eisenberg, professor of urology at Stanford medicine. It also refutes previous research that suggests penis sizes are getting smaller.

In a press release, Eisenberg said that it isn’t clear what may be causing the trend, but it’s concerning to see a significant change in such a short period of time. 

“If we’re seeing this fast of a change, it means that something powerful is happening to our bodies,” Eisenberg said in the press release. “Just as we measure height and weight every year across the U.S., 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.”

Potential factors could include chemical exposure to substances like pesticides and other environmental contaminants, the researchers wrote in the study. Some evidence suggests these chemicals are already affecting our bodies, disrupting hormones and potentially leading to earlier puberty and worse reproductive health. 

But previous research by epidemiologist Shanna Swan found these environmental factors to have the opposite effect, making penises smaller and lowering sperm counts. Swan concluded this from her own meta-analysis of male reproductive health studies, published in 2017.

These new study results are good reason for men to start being more aware of their own sexual health, urologist and men’s health specialist Dr. Justin Dubin told Insider. 

“Sexual health is a good reflection of overall health. There could be many reasons why penis size is changing,” he said. 

“It’s a thoughtful study, but one study doesn’t necessarily mean penises are getting bigger or smaller,” he said. “Hopefully it will have guys thinking about their health, which is often stigmatized, and talking to their doctors more often.” 

For one thing, penis length is notoriously hard to measure, pun intended. Factors like temperature and level of arousal can lead to inconsistent results, according to Dubin. Lifestyle also plays a role — weight gain can make a penis appear shorter, for instance, while habits like healthy diet and exercise are generally linked to better sexual health as well as overall health, he said. 

The takeaway for men here isn’t to get out a ruler — what matters is raising awareness of men’s sexual health, and encouraging further conversation, Dubin said.