If moderation on all cross paths could be dropped, an extended univariate model was then fit (Figure 2). It too includes β moderation parameters on each of the a, c, and e paths and moderation is tested in the same way as on the unique a, c, and e paths in the bivariate moderation model described above. The difference from the original Purcell univariate model is that estimation of the behavioral outcome value for each twin is corrected for both the value of twin’s moderator and their co-twin’s moderator. To assess moderation the β parameter was dropped on a, c, and e paths consecutively.
