**2.2 Schizophrenia as a heritable disease**

It has long been known that schizophrenia runs in families (McGue & Gottesman, 1991). Adoption studies demonstrate an increased risk of schizophrenia in biological relatives of adoptees with schizophrenia, suggesting that genetic components play an important role in the etiology of schizophrenia (Kendler & Dichl, 1993). Now it has been established by twin studies that heritability of schizophrenia is ~0.85 (Cannon et al., 1998; Cardno et al., 1999).

Although the mode of transmission of schizophrenia is still unknown, several reports suggest a higher maternal transmission of schizophrenia (Shimizu et al., 1987; Goldstein et al., 1990; Valero et al., 1998; Li et al., 2007).
