Mind & Body

A Doctor Cracked His Knuckles for 50 Years to See If It Was Harmful

When Dr. Donald L. Unger was a child, his mother and other relatives told him that cracking his knuckles would cause arthritis. Instead of just ignoring their warnings, Unger decided to take their hypothesis into his own hands. Literally.

The Knuckle Experiment

Dr. Ungler spent 50 years cracking the knuckles on only his left hand at least twice a day. He left his right hand alone as a control. In a tongue-in-cheek letter to the editor of Arthritis & Rheumatism, he estimated that he cracked the knuckles of his left hand at least 36,500 times, while the knuckles of his right were cracked "rarely or spontaneously."

At the end of his experiment, he examined his hands for arthritis and found none. "This result calls into question whether other parental beliefs, e.g., the importance of eating spinach, are also flawed," he amusingly concluded. "Further investigation is likely warranted." The entire letter has a similar tone. ("This study was conducted entirely at the author's expense," Unger clarifies in a note at the end — as if cracking your knuckles were a quite expensive lab process.)

The accolades Dr. Unger has received for this study have been as tongue-in-cheek as his findings. In 2009, his experiment won an Ig Nobel Prize, an award given to scientists for "achievements that first make people laugh, then think." He was in excellent company. 2017 Ig Nobel laureates include three physicists who "analytically determined why pregnant women don't tip over," and the inventors of a bra that doubles as a gas mask.

"Mother, you were wrong!" Dr. Unger exclaimed during his Ig Nobel acceptance speech.

What This Means for Knuckles Everywhere

Not a lot. Dr. Unger's experiment only had a sample size of one person (or two hands). However, it does highlight a relative dearth of recent research on knuckle-cracking — most of our data on its health effects is many decades old.

What recent research we do have, though, supports Dr. Unger's findings For instance, a 2011 study with a sample size of 215 found that there was no significant difference in the risk of hand osteoarthritis for people who cracked their knuckles and people who didn't. In December 2015, researchers also presented a study to the Radiological Society of North America that found no immediate differences in pain, swelling, flexibility, or grip strength between people who had and hadn't cracked their knuckles. This study had 40 participants — not quite 215, but it's still 40 times more robust than Dr. Unger's lighthearted study.

Get stories like this one in your inbox or your headphones: Sign up for our daily email and subscribe to the Curiosity Daily podcast.

If you want to read more about this subject, check out "Ask a Science Teacher: 250 Answers to Questions You've Always Had About How Everyday Stuff Really Works" by Larry Scheckel. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Ashley Hamer April 24, 2018

Curiosity uses cookies to improve site performance, for analytics and for advertising. By continuing to use our site, you accept our use of cookies, our Privacy Policy and Terms of Use.