Table 4 shows the phenotypic correlations between conduct symptoms and the three substance use variables as well as the cross-twin-cross-trait (CTCT) correlations (e.g. correlation of conduct symptoms twin 1 with alcohol use twin 2). We found significant modest to moderate phenotypic correlations between conduct symptoms in early adolescence and all three measures of substance use in young adulthood, ranging between 0.16 and 0.35. The CTCT correlations provide a first impression of the extent to which the phenotypic correlations between conduct symptoms and the substance use variables are due to A, C, and E (this is formally modelled below). For males, the CTCT correlations appear higher for MZ twins than for DZ twins for conduct-alcohol and conduct-cannabis, suggesting that overlapping genetic influences may explain the phenotypic correlation. The CTCT correlation for conduct-tobacco use for males and all female CTCT correlations appear comparable between MZ and DZ twins, suggesting that the phenotypic correlations between these variables may be more influenced by shared environmental factors.
