Personality disorders may have an important moderating effect on the success with which various kinds of mental disorders can be treated.  Specific examples include depression and eating disorders.  Some evidence indicates that both psychological and pharmacological treatments are less effective if the patient also qualifies for a diagnosis of at least one form of personality disorder. The issue demands serious attention because it should influence treatment planning. If co-occurring personality disorders do have an impact on the treatment success, then appropriate steps should be taken to address these problems.  On the other hand, if personality disorders do not affect the outcome of treatment, clinicians will be able to save time and aim their interventions exclusively at more acutely disturbing behaviors and problems for which people commonly seek help.

There are several reasons to believe that personality pathology could have an impact on the success of treatment.  People with various types of personality disorders would be expected to have difficulty establishing an effective working relationship with their therapist. The presence of a co-morbid personality problem may also diminish the size and quality of the patient’s social support network, and decreased social support is known to be associated with more negative long-term outcomes for various types of mental disorders. Finally, personality problems may increase the person’s risk for experiencing stressful life events, and these events may trigger further episodes of disturbed eating.

Relatively few studies have examined whether personality disorders and/or personality traits have a significant impact on the outcome of treatment for depression and eating disorders.  The evidence that is available is inconsistent.  We believe that inconsistencies in results between studies may stem largely from problems with the procedures used to measure PDs.  One problem is that most studies have relied exclusively on self-report tools for the assessment of personality disorders.  Our own research suggests that questionnaires and semi-structured diagnostic interviews have a number of serious limitations in this arena because people with personality disorders are frequently unable to view themselves realistically and sometimes are unaware of the effect that their behavior has on other people. In order to fully understand the connection between personality disorders and other forms of psychopathology, it is essential that research studies supplement the use of self-report measures with the collection of data from informants who are well acquainted with the patient.