We again simulated data for standard normally distributed moderator M and trait T in 500 MZ and 500 DZ twin pairs. Suppose again that A, C, and E account for 40%, 30%, and 30%, respectively, of the variance in M. The parts of the cross paths between M and T that do not depend on M (a c, c c, and e c in Fig. 1a) are all set to .05, and A, C, and E unique to T (a u, c u, and e u in Fig. 1a) are set to .35, .25 and .25, respectively. That is, if moderation is fully absent, the correlation between M and T equals .39, while genetic and (common) environmental effects explain 40%, 30% and 30% of the variance in T, respectively. We now introduce moderation on the cross paths by setting either β ac, β cc, or β ec to .10. Moderation on the unique parts of T is, however, absent (i.e., β a, β c, and β e in Fig. 1a are set to 0). For each of these settings we
