We analyze a population-based sample of Norwegian twins recruited from the Norwegian Institute of Health Twin Panel [31]. Approval for this study was received from The Norwegian Data Inspectorate and the Regional Committee for Medical and Health Research Ethics, and written informed consent was obtained from all participants after a complete description of the study. The sample has been used in many previous investigations [e.g., 24,32]. In wave 1, lifetime history of major DSM-IV Axis I disorders and all the 10 Axis II PDs in past 5 years were assessed at interview in 2801 twins (43.5% of those who were eligible; 1390 complete twin pairs and 21 single twins), between the years 1999 and 2004. Their mean age was 28.2 years and age range 19 to 36. A previous analysis of attrition bias indicated that, despite moderate selection towards good mental and somatic health, attrition does not appear to affect twin analyses of mental health [33]. In wave 2, altogether 2284 twins (987 complete pairs and 310 single twins) were re-interviewed approximately 10 years later. Attrition from 1st to 2nd
