Contrast effects or genetic dominance (non-additivity) pose alternative explanations for the very low DZ correlations relative to MZ correlations observed in the inattention and hyperactive/impulsive data. To account for this, an additional path between INATT scores of each twin and HYP/IMP scores of each of the twins, s, was also added to the model (not shown in Figure 1). This path implies an interaction between phenotypes, and may be interpreted in two ways (Simonoff et al., 1998): (1) a social interaction between siblings (i.e., the behavior of one twin has an effect on the behavior of his/her cotwin) that can be either cooperative or competitive; or (2) a rater effect (i.e., parents stress the similarities or differences between the children). Following Rietveld et al. (2003), this will be referred to as a ‘contrast effect’ for the remainder of this article.
