Cracking your knuckles causes arthritis
The old claim turned out to be wrong and was rejected by evidence.
TaughtStop cracking your knuckles or you will get arthritis when you are older.
NowNo association has been found. A doctor cracked the knuckles on one hand only for over 60 years as a personal experiment and found no difference; a 2011 study of 215 people found no link either.
What actually happened
Donald Unger deserves the credit here. Irritated by his mother's warnings, he cracked the knuckles of his left hand at least twice a day for more than sixty years while never cracking the right, then reported the result: no arthritis in either hand, no difference between them. He was awarded the 2009 Ig Nobel Prize in Medicine.
Proper studies agree. Kevin deWeber and colleagues examined 215 people in 2011 and found no correlation between knuckle cracking and hand osteoarthritis. The sound itself was settled by MRI in 2015: it is a cavity forming rapidly in the joint fluid as the joint separates, which is why you have to wait before you can do it again. The honest caveat is that at least one older study associated heavy long-term cracking with reduced grip strength and hand swelling. Nobody has connected it to arthritis.
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Who was taught this
Still standard through 2015, so anyone who finished school between 1950 and 2011 learned the earlier version.