In contrast with the regressions on the true latent trait, the picture changes markedly when analysis is based on test scores for the digenic additive model. For example, even when data are simulated under the simplest additive model (Table 5), regression analysis of the symptom counts, S, derived from a test with equal item parameters (“Test 1”) yields a much less parsimonious model, in which not only homozygous effects of both loci are significant (though less so) but there are also marked non-additive genetic effects, including some dominance and strong additive × additive epistasis. Furthermore, raw symptom counts yield highly significant evidence of homozygous effects on sensitivity to the environment: (βda, βdb) = (0.423, 0.442) and even evidence of higher order interactions with apparent epistatic effects contributing to G × E: (βiab, βjab, βjab) = (−0.164, 0.160, 0.164).
