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 the warnings, Unger took a more scientific approach: he 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 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 concluded. "Further investigation is likely warranted." In 2009, Dr. Unger's experiment won an Ig Nobel Prize, an award given to scientists for "achievements that first make people laugh, then think."
Of course, this is just one experiment with a sample size of one. Though there has been research into the health effects of knuckle cracking, most of it is many decades old. In December 2015, however, researchers presented a study to the Radiological Society of North America that found no immediate differences in pain, swelling, flexibility, or grip strength between groups of people who had and hadn't cracked their knuckles. Debunk more schoolyard body myths with the videos below.