Historically, health professions have not always agreed on how best to conceptualise, categorise and define personality disorders. Although there are still many divergent views, there has been an increased consensus following the publication of definitions of personality disorder in ICD-10 (World Health Organization, 1992) and DSM-IV (American Psychiatric Association, 1994). In 2003, the Department of Health, in conjunction with the National Institute for Mental Health in England, produced Personality Disorder: No Longer a Diagnosis of Exclusion (National Institute for Mental Health in England, 2003). This document outlined the need for provision of mental health services (both general and forensic) for people with a diagnosis of personality disorder, and emphasised the importance of practitioners having skills in identifying and assessing personality disorder in order to appropriately treat a person's difficulties.

Prevalence and comorbidity of personality disorders

Personality disorders are common conditions (Coid et al, 2006a) that, by definition, run a prolonged course and are often associated with poor outcome (Stone, 1993; Skodol et al, 2005) and increased mortality (Harris & Barraclough, 1998). In a general population study of British households, Coid et al (2006a) found a weighted prevalence of 4.4% for a diagnosis of any personality disorder. The weighted prevalence for each individual personality disorder varied between 0.06 and 1.9%, with obsessive–compulsive, avoidant, schizoid and borderline subtypes being the most common. Dependent and schizotypal personality disorders were the least prevalent (the study identified no cases of histrionic or narcissistic personality disorder, suggesting that these disorders are particularly rare in the general population). Comorbidity within personality disorders is common; thus, patients with personality disorder are likely to fulfil diagnostic criteria for more than one subtype of personality disorder. In Coid et al's (2006a) sample, 54% had only one personality disorder, 22% had two, 11% had three and 14% had between four and eight personality disorders.