Video via Vox: What's actually happening when you crack your knuckles? Was your mom right when she told you it would cause arthritis? The article includes information on Donald Unger, winner of an Ig Nobel prize and probably the leading self-tester on the subject:
Consider the findings of Donald Unger, arguably the most dedicated researcher of knuckle cracking. In his teens, after his mother warned him the habit would give him arthritis, he started cracking his knuckles in the name of science, according to a 2009 Los Angeles Times story. He continued to pop the joints in his fingers for over 60 years — but only on his left hand. He left the right hand untouched as a comparison. And his decades-long self experiment found no arthritis in either hand.