These HMR models were constructed to predict ERP value in 1 twin (twin A) from the ERP value of the co-twin (twin B), that pair's zygosity; the alcohol use for twin A, and the two-way cross-product terms of zygosity and twin B ERP value, zygosity and twin A alcohol use, and twin B ERP value and twin A alcohol use; finally, the three-way interaction term of zygosity, alcohol use, and twin B ERP value was obtained. The analyses yielded significance tests for genetic effects and evaluated the modulation of such genetic effects by the ERP value of the co-twin and alcohol use. Furthermore, all models included the main effects of smoking status and gender. Table 3 specifies the hierarchical models and interprets their meaning when significant. Note that the main effects of zygosity, gender, smoking and alcohol use, and interactions among them can affect only the ERP values of individual twins, rather than the pair-wise similarity in ERP values of twin pairs. Determining such effects are not, accordingly, our primary goal. Instead, we target interactions that influence intrapair resemblance, and,
