Although the distributions of the variables reasonably approximated normality, some variables remained mildly-moderately skewed and/or kurtotic following transformation. Therefore, biometric models were fitted by method of maximum likelihood with robust standard errors (MLR) to the raw twin data using Mplus version 7 (Muthén & Muthén, 1998–2012). Univariate models were constructed to partition the variation in the ages of tobacco, alcohol, and cannabis initiation – each considered continuously – into additive genetic (A), shared environmental (C), and unique environmental (E) influences, and test for quantitative sex differences. Evidence for sex differences was tested by comparing the fits of models that allowed parameter estimates for men and women to vary with the fits of models that equated estimates. Because the MLR estimator was employed, nested models were compared using the Satorra-Bentler scaled chi-square difference test.
