I think it's a combination of how much they shed as well as how much they groom - after all, they pick up the hairs when they lick. I've been very, very lucky in that respect. I've only had one cat that had hairballs. He was one of my only two longhaired cats, but I also had a compulsive groomer who did not have hairballs. Even Cosmo, my one guy who did have them, didn't seem to have much of a problem, maybe because he coughed them up frequently - at least once a day. Actually, I never witnessed him coughing them up; I would just come across his little "Cosmettes" that he left behind on the carpet.