Measures

Personality disorders were assessed using a Norwegian version of the Structured Interview for DSM-IV Personality (Pfohl et al. 1995). The method was initially developed in 1983, and has been used in a number of studies in many countries including Norway (Torgersen et al. 2001; Helgeland et al. 2005). It is a comprehensive semi-structured interview of all DSM-IV Axis II diagnoses, rating the specific DSM-IV criteria according to following guidelines: 0 = not present or limited to rare isolated examples; 1 = subthreshold (some evidence of the trait, but not sufficiently pervasive for the criterion to be considered present); 2 = present (criterion clearly present for most of the time during last 5 years); 3 = strongly present (associated with subjective distress or some impairment in social or occupational functioning or intimate relationships). The criteria were modeled based on an inferred ordered continuous threshold liability model of the endorsed ordinal category frequencies (e.g. polychoric correlations); to lessen the impact of empty cell conditions, the ordinal classes 2 and 3 were collapsed into a single class.

Most of the interviewers were psychology students in their final part of training or experienced psychiatric nurses. They were trained by professionals (1 psychiatrist and 2 psychologists) who had extensive previous experience with the instrument, and they were closely followed up individually during the entire data collection period. Most of the interviews were conducted face to face, but for practical reasons, 231 (8.3%) were obtained by telephone. Each twin in a pair was interviewed by a different interviewer. Interrater reliability was assessed based on 2 raters’ scoring of 70 audiotaped interviews: intra-class correlation of 0.91 for the number of endorsed ASPD criteria at the subthreshold level has been reported (Torgersen et al. 2008).