In the same way, the cross-twin cross-trait correlations (CTCT; e.g. the correlation between conduct problems of twin 1 and cannabis use of twin 2) are used to partition the covariance between conduct symptoms and use of tobacco, alcohol, and cannabis into A, C, and E influences. When the CTCT correlation is higher for MZ twins than for DZ twins this indicates that part of the phenotypic correlation between the two traits is due to overlapping genetic influences. A parallel logic follows for the interpretation of C and E paths that contribute to the covariance between the traits. To model this, we fitted a multivariate Cholesky decomposition with the four phenotypes, in which we estimated A, C, and E contributions to each of the variables as well as the A, C, and E contributions to the phenotypic covariation between the phenotypes (See Figure 1). In this way we obtained a measure of overlap in the genetic and environmental variation underlying conduct symptoms during adolescence and the use of substances in young adulthood.
