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Original Articles
Introduction
Cross-sectional studies have shown that genetic factors contribute substantially to risk for cluster A personality disorders (PDs) and associated traits (Claridge & Hewitt, 1987; Kendler et al. 1987; Kendler & Hewitt, 1992; Torgersen et al. 2000; Linney et al. 2003; Jang et al. 2005; Parnas et al. 2005; Kendler et al. 2006). While prior twin studies have suggested, with considerable consistency, that genetic influences on normative personality traits in adulthood are relatively stable over time (McGue et al. 1993; Viken et al. 1994; Blonigen et al. 2008; Bleidorn et al. 2009; Kandler et al. 2010), we know little about the stability of genetic influences on pathological PDs and traits.
A prior cross-sectional twin study of the three cluster A PDs [schizotypal PD (STPD), paranoid PD (PPD) and schizoid PD] showed that these three PDs, and especially STPD and PPD, share common genetic and environmental risk factors (Kendler et al. 2006). In this study, we report on a longitudinal twin study of two of these cluster A PDs - STPD and PPD - assessed at personal interview in a population-based twin sample twice, 10 years apart. We examine these two disorders assessed at two times using a single-factor model that treats STPD and PPD as indicators of a common underlying cluster A vulnerability.
We are particularly interested in determining the stability of the genetic and environmental influences on these two PDs, and to what extent this stability is the result of factors that reflect the latent liability shared between the two PDs versus factors specific to the individual PDs.
Method
Sample and assessment methods
Twins were recruited from the Norwegian Twin Registry (NTR) at the Norwegian Institute of Public Health. The twins were originally identified through the Medical Birth Registry of Norway (MBR). The MBR was established 1 January 1967, and receives mandatory notifications of all births. The twins born 1967-1979 (n = 15 374) were invited to participate in questionnaire studies in 1992 and in 1998. Altogether, 12 700 people received the second questionnaire and 8045 twins responded after one reminder (response rate 63%). Zygosity of the twins was determined by the use of questionnaire items for the entire sample and by microsatellite markers...