Lesch-Nyhan is at the far end of a spectrum of self-injurious behaviour. You and I and everyone who is human, we all do things which are bad for us. We’ll sit down in front of the television and eat a quart of ice cream. We all have self injurious impulses, too. Driving a car, we can have a strange impulse to drive on the wrong way and smash into something. Many people bite their fingernails. They’ll tell you it’s gross and that they don’t want to do it — ‘Sometimes I get nervous and start biting my fingernails,’ they’ll say. There are people who chew their lips nervously. Now let’s turn up the volume a little: some people bite their cuticles. Turn up the volume a little more: some people bite their cuticles until they bleed. Now let’s turn the volume way up. Now you have someone biting off tissue and bone in his fingers, biting off the whole finger, and chewing his lips off. Where, in this spectrum of behaviour, is free will?
—H.A. Jinnah, quoted in Panic in Level 4 by Richard Preston, p.170.