Before estimating the variance components, the effects of sex, age and zygosity on the thresholds of each variable were tested using an α-level of 0.01. Levels of conduct symptoms and substance use did not significantly differ between MZ and DZ twins in either sex. We found a significant sex effect on the thresholds for conduct problems, alcohol and tobacco use (all p<0.001, respectively), indicating that males report more conduct problems and use more alcohol and tobacco than females. No significant sex effects were found for cannabis use, suggesting no differences in levels of cannabis use between sexes. No significant age effects were found for any of the variables. In subsequent genetic analyses thresholds were estimated separately for males and females and age was included as a covariate.
