A young person with anorexia nervosa, for example, cannot be persuaded to maintain a minimally normal body weight. This child or adolescent is intensely afraid of gaining weight and doesn't believe that he or she is underweight. Anorexia affects 1 in every 100 to 200 adolescent girls and a much smaller number of boys. Afterward, to prevent weight gain, they rid their bodies of the food by vomiting, abusing laxatives, taking enemas, or exercising obsessively. Reported rates vary from 1 to 3 out of 100 young people.
Autism spectrum disorder or autism appears before a child's third birthday.
Children with autism have problems interacting and communicating with others. They behave inappropriately, often repeating behaviors over long periods. For example, some children bang their heads, rock, or spin objects. The impairments range from mild to severe. Children with autistic disorder may have a very limited awareness of others and are at increased risk for other mental disorders.