There is no single known cause of any eating disorder, but they exist largely in affluent cultures where there is an abundance of food. No one knows why voluntary behaviors, such as eating smaller or larger amounts of food than usual, morph into an eating disorder for some people.
Biology is a factor: Appetite control and the regulation of food intake is very complex, with many hormones in the brain and the body contributing to signals of hunger and satiety.
Culture is thought to play a significant role, as women, especially, are pressured to fit an ideal of beauty that is increasingly defined by the sole criterion of body weight. Families contribute, too; parents who emphasize looks and/or dieting or criticize their children's bodies are more likely to raise a child who develops an eating disorder.
Other circumstances figure in, too; the conditions can be triggered by stress, social difficulty, loneliness, depression and other unpleasant emotions, or dieting itself.