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Introduction
Paranoia is a central symptom of psychosis. Delusions of reference and delusions of persecution occur in at least 50% of cases of schizophrenia (Sartorius et al. 1986; Cutting, 1997). However, paranoid thoughts also occur in the non-clinical population (e.g. Verdoux et al. 1998; Peters et al. 1999; Martin & Penn, 2001; Freeman et al. 2005a, c). At least 10-15% of the general population regularly experience paranoid thoughts (Freeman, 2007). Increasingly there are studies of paranoia across the continuum of severity (see reviews by Bentall et al. 2001; Freeman, 2007; Garety et al. 2007). However, current research lacks a measure that is both specific to paranoid thoughts and reliable and valid for both clinical and non-clinical groups.
Clarifying a definition of 'paranoia'
Single-symptom research requires confidence that researchers are isolating the same elements of psychopathology and thus clear definitions of the phenomenon of interest are needed. This has been addressed specifically for persecutory delusions by criteria proposed by Freeman & Garety (2000). For individuals to hold persecutory beliefs, they must believe that harm is occurring, or is going to occur to them and that the persecutor has the intention to cause that harm. Ideas of reference are specifically excluded because they do not contain the element of intention to harm. Nonetheless ideas of reference and persecution are often related. It has been suggested that a 'hierarchy' of paranoia exists, extending from mild social evaluative concerns, through ideas of social reference, to persecutory beliefs concerning, mild, moderate and severe threat (Freeman et al. 2005c). Startup & Startup (2005) have defined delusions of reference by themes of observation (ideas relating to surveillance and gossip of the individual) and communication (ideas relating to the communication of information about the individual, e.g. through hints, double meanings, on the TV, newspaper headlines, radio, etc.) and found that observation beliefs (i.e. ideas relating to more interpersonal concerns), were particularly associated with persecutory delusions. It is therefore important that persecution and social reference are considered within the spectrum of 'paranoia'.
Existing measures of paranoia
There are a number of questionnaire assessments of paranoia, but the most widely used is the Fenigstein & Vanable (1992) Paranoia Scale (PS). It was...