Twin studies allow researchers to disentangle genetic and environmental influences in psychiatric disorders. Because monozygotic (MZ or identical) twins share 100% of their genes, and dizygotic (DZ or fraternal) twins share, on average, 50% of their genes, comparisons between MZ and DZ twins are conducted to evaluate the degree of genetic and environmental influence on a specific trait and to calculate heritability estimates. Higher concordance between MZ twins than DZ twins, indicates a stronger genetic influence on a trait or disease, whereas similar concordance between MZ and DZ twins suggests greater environmental influences. Path analytic or variance component approaches that estimate the proportion of variance attributable to additive genes, common and unique environment have been the standard method of analysis of data from large twin studies. The combined twin family design is an even more powerful design that yields estimates of heritability and permits evaluation of multigenerational patterns of expression of genetic and environmental risk factors. 96
