The longitudinal model included all five zygosity groups, allowing for the examination of quantitative and qualitative sex differences. Quantitative effects examine whether the magnitude of genetic or environmental effects differs between the sexes (Kendler & Prescott, 2006). Qualitative effects examine whether the same genes are involved in the etiology of substance consumption in the sexes and are measured by the genetic correlation (rg). This correlation reflects the degree of resemblance between the genetic risk factors for males and females and can vary from zero (i.e. entirely distinct set of genes) to one (i.e. identical genetic factors). When examining for sex differences it is assumed there is only one correlation structure for both sexes, but males and females may have different loadings on these factors (Neale et al. 2006). However, the Choleksy decomposition allows for different correlation structures to be estimated for males and females. Thus, a constraint was added to constrain male and female correlation structures to equality (Carey, 2005; Neale et al. 2006).
