Twin studies in psychotic disorders

Abstract

etiology of major psychoses, particularly schizophrenia and bipolar disorder. Nevertheless, little is known about the nature and extent of the specific genetic contribution to disease liability (Kendler et al., 1993; Danese, 2008). Twin studies can provide crucial insights regarding the etiology of sub-threshold and clinical psychosis, and represent an extraordinarily powerful design to establish the role of genes and environment both in the expression of a trait (e.g., development of a disease) and in the co-expression of multiple traits (e.g., comorbidity of multiple diseases). By comparing the correlation for a trait between genetically identical monozygotic (MZ) twins with that between dizygotic (DZ) twins who represent full sibs, and assuming that MZ twins share relevant environmental exposures to the same extent as DZ twins (‘equal environments assumption’), it is possible to estimate the contribution of genetic factors (“heritability”) and environmental influences to the expression of the trait. Furthermore, when multiple phenotypes are studied, cross-twin/cross-trait correlation (i.e., between a given phenotype in a twin and another phenotype in his/her cotwin) in MZ and DZ twin pairs provides information on genetic or environmental influences common to the phenotypes (genetic or environmental correlation between the phenotypes), which could play a role in their coexpression (Boomsma et al., 2002; Spector et al., 2000). The potential of the twin design enormously increased after the implementation of population-based registries of data on twins in several countries (Busjahn & Hur, 2006), including Italy (Fagnani et al., 2006; Stazi et al., 2002). A large body of twin studies aimed at unraveling the genetic and environmental architecture of major psychoses and psychotic symptoms. Although results vary Twin studies in psychotic disorders