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Original Articles
Introduction
Delusional disorder (DD) was originally described in the 19th century by Kraepelin (1915) as a distinct illness with well-systematized delusions that were not bizarre, which he termed paranoia. Winokur (1977) further defined the characteristics of DD as prominent non-bizarre delusions in the absence of any accompanying hallucinations. The term 'delusional disorder' has since been introduced into diagnostic classification systems including DSM-III-R and DSM-IV, and under the term persistent delusional disorder in ICD-10. Recently, DSM-V removed the requirement of non-bizarre delusions in its definition of DD, instead adding a delusion bizarre-type specifier to provide continuity with DSM-IV. Previous research, which mainly focused on examining the profile of DD, found those with DD were more likely to be female and married, have little occupational impairment, and high co-morbidity (Munro & Mok, 1995; Yamada et al. 1998; Grover et al. 2007). However, only a few systematic studies have examined if DD has a distinct aetiology separate from other psychoses and affective disorders (Kendler, 1982; Marneros et al. 2012). The lack of studies could be due to low DD prevalence, and features of the disorder such as high functioning and lack of insight might limit the ascertainment of an adequate sample size (Ibanez-Casas & Cervilla, 2012). The well-known review of 17 studies by Kendler (1982) suggested DD was distinct from paranoid psychosis as it was associated with an older age of onset, shorter hospitalization, being native-born and socially disadvantaged, as well as being female and married as mentioned above. However, these studies were conducted before the introduction of standardized diagnostic criteria. Later studies that used DSM-III (Fabrega et al. 1992) and ICD-10 classifications (Jager et al. 2003) reported similar findings for the characteristics of DD, except they found there was better social functioning. These findings suggest the possibility that DD could be differentiated from schizophrenia (SZ).
In the three decades following Kendler's review, there has only been one large recent comparative longitudinal follow-up study on a cohort of 43 DD patients and 42 paranoid SZ patients. This study concluded each disorder was an independent and separate entity that exhibited significantly dissimilar symptoms, course, and outcome (Marneros et al. 2012). In addition to demographic differences, they...