VATSPSUD includes data collected from Caucasian twin pairs born between 1934 and 1974. Details on ascertainment and data collection procedures have been described previously (Prescott and Kendler, 1999; Prescott et al., 2004) and are not elaborated here. The current study is based on data obtained at the most recent wave of interviews, which is wave 4 in the female–female sample (FF4) and wave 2 in the male–male and male–female sample (MF2). We used data from the FF4 and MF2 waves because they provide detailed SLE data on the largest number of individuals. The structure of the FF4 and MF2 interviews for assessing SLE was slightly different (see Measures), so the SLE prevalence from the two may not be directly comparable. We thus conducted all analyses separately by sex and excluded female twins in MF pairs, because we could not validly equate the SLE means and variances of the 2 female groups as would be required if we included MF pairs in the twin analyses.
