"There is no single cause of an eating disorder. We know that genetics play a large role, but genetic vulnerability is only part of the story. Environment plays a role too, especially in triggering onset, which often occurs in adolescence. Pressure to diet or weight loss related to a medical condition can be the gateway to anorexia nervosa or bulimia. For those who are genetically vulnerable to anorexia nervosa, once they lose the first five to 10 lbs, dieting becomes increasingly compelling and rewarding. Looked at another way, if eating disorders were the result solely of social pressure for thinness we would expect eating disorder rates to have increased as obesity has in the past few decades, yet anorexia nervosa and bulimia remain relatively rare and often cluster in families."