According to the “bio-psycho-social model”, attention deficit hyperactivity syndrome is caused by the interaction of physical, psychological and social factors. On the physical level, a disturbance in the metabolism of the neurotransmitter “dopamine” in the brain is mainly considered. There also seems to be a genetic predisposition as studies of siblings and twins have shown. Mental illness in parents, particularly personality disorders and alcohol dependence, also contribute to an increased risk of ADHD in children. A number of social factors appear to favor the development of attention deficit hyperactivity disorder. These include difficulties within the family, low income, cramped living conditions, and cigarette or alcohol consumption by the mother during pregnancy. However, the disorder only occurs in the interplay of all these factors, which is why these factors are present more often than average in those affected, but do not necessarily have to be present for ADHD to develop.