large correlations. In case the true DZ correlation equals half the true MZ correlation, DZ:MZ = 1:2, the correlations of the sum scores will show a smaller ratio, DZ:MZ < 1:2, usually an indication of dominance genetic effects or epistasis. This is hard to see from the Fig. 1D, but with 5 items, the simulated sum score correlation is 0.83667 when the true correlation is 0.9 (92.96%), 0.42429 when the true correlation is 0.5 (84.86%), and 0.076575 when the true correlation is 0.1 (76.58%). Suppose we could analyse the true correlations, 0.9 and 0.5. One would then conclude that additive genetic variance accounts for 80% of the variance, non-shared environmental effects 10% and the shared environmental effects for the remaining 10%. Now if we would base our analysis on the observed sum score correlations 0.84 and 0.42, we would conclude that there are no shared environmental effects. One can imagine that when the true correlations are 0.90 and 0.45 one would conclude dominance effects to be absent, whereas if one would analyse observed sum scores correlations, one would find evidence for dominance genetic variance, the extent of which is dependent on the number of items.
