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 large fraction of their environment, and typically more so than other siblings or relatives. If there is a genetic contribution, the concordance rate for a genetically affected disorder should be higher for MZ than for DZ twins. Almost all of the twin studies of schizophrenia conducted to date have shown a higher concordance rate in MZ twins (40–65%) than in DZ twins (0–28%) which indicates a clear genetic component to its etiology (Cardno et al., 1999; Cardno and Gottesman, 2000; Kringlen, 2000). Further evidence that schizophrenia is genetically transmitted has been obtained from adoption studies which have shown that the biological relatives of schizophrenia patients have a higher risk of developing the disorder (Lowing et al., 1983; Kendler and Gruenberg, 1984; Kety, 1987; Kendler et al., 1994; Kety et al., 1994; Tienari et al., 2000, 2003). However, adoption studies have also demonstrated that the adopted-away children of mothers with schizophrenia only have a higher risk of developing the disorder when
