In model 1a the variances were constrained to be equal (for continuous data only). In model 2 the correlation in DZM pairs was constrained to the correlation in DFZ pairs (rdzm = rdzf) while in model 3 those correlations were also constrained to the correlation in dizygotic opposite-sex twin pairs (rdzm = rdzf = rdos). Testing of models was done by likelihood-ratio tests, by subtracting the negative log-likelihood (−2LL) for the more restricted model from the −2LL for the more general model. This yields a statistic that is distributed as chi square with degrees of freedom (df) equal to the difference in the number of parameters in the two models. If the difference test is significant (p<0.05) the constraints on the nested model cause a significant deterioration of the model.
