All modeling was conducted using the raw data option in Mx (Neale, 2000). Mx is a structural equation modeling program developed specifically for the use of twin and family data. When the outcome is ordinal (such as our measure of alcohol use), the model involves the use of thresholds, rather than means. Both quasi-continuous moderating variables were standardized for analyses. The first application of the moderation model using quasi-continuous environmental moderators was to the study of socioregional factors on alcohol use among young adults using an older Finnish twin sample (Dick et al., 2001). These models have subsequently been detailed and expanded (Purcell, 2002). The significance of each of the parameters in the model can be tested by dropping a parameter and evaluating the change in −2 log likelihood between the initial model and the nested submodel. This difference is evaluated using a chi-square distribution. A significant change in fit between the models (p < 0.05) for the difference in degrees of freedom indicates that dropping the parameter caused a significant decrease in fit of the model, indicating that pathway
