That's true for bulimics, too. Girls are taught early on to be good and to please other people. One way to please is to be thin. Girls are especially vulnerable in early adolescence. According to psychologist Carol Gilligan, girls show a drop in self-esteem and an increase in depression and eating disorders at that time.
Boys are most vulnerable to stress earlier in childhood.
As their breasts and hips get fuller with maturing, girls see that as getting fat. Since they have learned to judge themselves by what they think others want, they now try to shape themselves to what they think will please others. They are further pressured by the attention they get from boys. Along with a changing body comes a feeling that something is taking place over which the young girl has no control. Self-starvation is a way to feel in control again.