Maximum likelihood estimation techniques (Neale and Cardon 1992) were used in Mx (Neale et al. 2003) to calculate within-trait and cross-trait correlations and conduct univariate and bivariate (Fig. 1) behavior genetic models examining maltreatment and conduct problems. Raw ordinal data were analyzed, and age- and sex-specific thresholds were included as definition variables. In analyzing ordinal data, Mx uses the multinormal threshold maximum likelihood fit function, which is described in detail in Neale et al. (2003).
