It is a complex condition. Doctors don't know. We have some clues, though.
Some examples are children who are born premature and children who are injured in the birth process.
Probably it's a brain disease. Doctors think it involves links between nerve cells in the brain. It has to do with chemicals in the brain like epinephrine and dopamine. There is no simple test. Making the diagnosis depends on a team effort by parents, teachers, mental health professionals, and your child's physician.