Although the scientific study of personality disorders is at an early phase, research provides some preliminary answers for a number of the questions raised in this chapter. First, in addition to diagnostic specification by the DSM-IV criteria, personality disorders can be parsimoniously characterized with a limited number of continuous trait dimensions using psychometrically sound instruments (Livesley et al., 1992). Second, healthy personality can also be characterized in terms of a limited number of continuous dimensions using psychometric instruments. Third, the content domain of personality disorder symptoms and personality traits overlaps and can be characterized in terms of four independent traits: introversion, neuroticism, disagreeableness, and compulsivity (but not necessarily openness). Fourth, preliminary research on physiological correlates of personality disorder symptoms suggests that low serotonergic function may be associated with neuroticism and/or disagreeableness, while low dopamine function may be associated with introversion. Fifth, serotonergic interventions appear to affect symptoms related to neuroticism and/or disagreeableness, while the effects of dopaminergic interventions on introversion have received less characterization, both in personality disordered and healthy samples.

While these answers provide some clarification, they also raise further questions. Instead of new theories or measures, perhaps what is needed most at the present is integration across different levels of analysis (Depue and Collins, 1999; Knutson et al., 1998). Despite the incompleteness of our knowledge regarding the description, causes, development, and treatment of personality disorders, the advent of psychometrically sound measures of personality disorder symptoms can provide an anchor for integrative analysis. These symptom-focused measures may also provide optimal endpoints for elucidating physiological mechanisms that can promote, perpetuate, and even prevent personality disorders.

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