The fact that the rates of concordance for schizophrenia among monozygotic twins is far below 100% proves that non-genetic factors must be operant for the development or not of the disease. Such factors have frequently been discussed as related to the environment and were recently summarized elsewhere (Häfner 2002). They comprise obstetric complications, maternal psychopathology during pregnancy, intrauterine factors such as growth retardation, viruses, nutrition and low birth weight. Moreover, the risk of psychosis was also found to be influenced by factors such as urbanicity and migration respectively, two environmental factors that are complex because they comprise a series of elements such as culture, socioeconomics, infection and pollution.