Knuckle cracking causing arthritis is an old wives tale brought about by sheer coincidence. To date there's no actual evidence to suggest that it does and scientists have discredited the belief. In fact, one scientist conducted a life-long experiment in which every day he cracked all the knuckles in one hand and not the other, and to the day he died he never had arthritis. Arthritis is entirely caused by genetics and is passed down similarly to other traits, like hair colour or earlobes. However! Cracking knuckles may cause you to have a weak grip later on in life. There seem to be conflicting beliefs about this but generally speaking, I'd advice against knuckle cracking.
It's a different story with everyone. Some people say it can cause arthritis, and some say that that is a myth. I crack my knuckles, although I'm trying to stop because my hands keep cramping up. Please don't comment and say it's arthritis or early stages of it because I am sure it's not.