All the above datasets were generated on the assumption of no G × E interaction in liability yet all provide strong evidence of non-additive effects when subjected to the vagaries of psychological testing. The final data set (Table 9) explores the consequences of simple digenic G × E in which the main effects of both loci are homozygous (only da = db > 0) and both loci show homozygous differences in their linear response to the environment (βda = βdb > 0). If the true scores are known, the parameter estimates of the full model, including GE and epistasis, correspond to those of the underlying genetic architecture. Two further “wrong” models were fitted to the true scores to illustrate the possible biases that ensue from model misspecification. Omitting the two homozygous effects on G × E leads to grossly inflated estimates of the main effects. Allowing one locus to affect the average response and the other to affect G × E (da > 0, db = 0, βda = 0, βdb > 0) leads to biased estimates of both genetic
