What causes anorexia nervosa?

Unknown. The exact cause of anorexia nervosa is unknown. As with many diseases, it's probably a combination of biological, psychological and environmental factors. There may be genetic changes that make some people more vulnerable to developing anorexia. Some emotional characteristics may contribute to anorexia. Peer pressure may help fuel the desire to be thin, particularly among young girls.
Cause is unknown. No one knows, really. But two things we do know: parents do not cause anorexia nervosa in children and children do not choose to have it. Basically, an is a highly heritable brain disorder of unknown cause.