The twins participating in this study came from the Mid Atlantic Twin Registry, and were interviewed as part of the Virginia Adult Twin Study of Psychiatric and Substance Use Disorders (Kendler and Prescott, 2006). Data were collected on male–male and male–female twin pairs (MM/MF) at 2 separate interview waves. To be eligible for participation, twins had to be Caucasian and born between 1940 and 1974. Of the 9,415 twins who were eligible at the first wave, 6,812 (72.4%) completed interviews primarily by telephone. Approximately 1 year later, the twins who had participated in the first wave were contacted for a follow-up interview. The median time gap between the 2 interviews was 15.3 months (mean = 19.0; SD = 8.7). Telephone and face-to-face interviews were complete for 5,621, or 82.5%, of those eligible. At the time of the first interview (1993 to 1996), the subjects had a mean age of 35.1 years (SD = 9.1; range = 18 to 56), and had in average 13.4 years (SD = 2.6) of education. (See Kendler and Prescott, 2006, for a more detailed description of the sample.)
