A large number of studies have demonstrated that schizophrenia runs in families and that the average life time risk for first degree relatives is 10% (compared to a life time prevalence of approximately 1% in the general population). Certain features show particularly high familiality, such as age and mode of onset, the course of the disorder, the degree of impairment during the disorder, premorbid functioning, psychomotor deficiency, disorganization, and manic features (Wickham et al., 2001, 2002). Family studies are able to demonstrate that a disorder, or a specific symptom of a disorder, runs in families. However, they can not distinguish whether the familial aggregation is due to environmental or genetic factors since family members share some of their genes as well as many elements of their environment. In order to separate environmental from genetic factors, twin and adoption studies are conducted. Twin studies compare the rate of concordance between monozygotic (MZ) twins and dizygotic (DZ) twins. On average, DZ twins share 50% of their genes, as is the case with other non-twin siblings. Both kind of twins normally share a
