I'm not a knuckle cracker myself. In fact, I hate it when people crack their knuckles. And I've frequently told people that cracking their knuckles would cause arthritis. After all, that's what everyone says. But according to this NY Times article (republished in the Arizona Republic) it's not true. It's an urban legend.
Just reading this description of what causes knuckles to crack makes me cringe:
The loud pop of a cracked knuckle is caused by synovial fluid, the thick lubricant that surrounds every joint. When the fingers are stretched or bent backward, the bones of the joint pull apart. This creates bubbles of air in the fluid, which subsequently burst.
But as for the evidence that knuckle cracking doesn't cause arthritis, the article cites a 1990 study:
The largest study to explore a link to arthritis was published in 1990 in the Annals of the Rheumatic Diseases. It looked at 300 healthy people older than 45, 74 of them habitual knuckle crackers. The rates of arthritis of the hand were similar in both groups, though the knuckle crackers, on average, had reduced grip strength.
Still, I think I might keep telling people who are cracking their knuckles that it's going to give them arthritis, just to annoy them and maybe scare them into stopping.