We conducted 3 sets of twin-pair analyses to estimate the sources of genetic and environmental overlap between onset age, PYDD (after partialing the effects of demographic covariates), and each type of SLE: totSLE, dSLE, and iSLE. For all 3 types of SLE, eliminating shared environmental influences resulted in minimal loss of fit relative to the full model (Δχ2 < 0.1, Δdf = 1 for both men and women). Therefore, a reduced model of genetic and individual environmental influences (AE model) was used as the baseline for comparing subsequent nested models.
