Our analytic approach involved three major steps: 1) estimating polychoric correlations for 44 (2×22) variables, including within-twin cross-disorder, cross-twin within-disorder, and cross-twin cross-disorder correlations, for monozygotic and dizygotic twins separately; 2) estimating genetic and environmental correlations between all 22 disorders, based on multivariate biometric modeling; and 3) applying exploratory factor analysis to the resulting genetic and environmental correlation matrices.
