Understanding the influence of patients’ and therapists’ personality on psychotherapy treatment outcomes

The term personality refers to stable patterns of thinking, feeling, and behaving that are unique to each individual. Due to the universal influence of personality traits on a range of human activities and life outcomes, researchers have considered whether personality is relevant in the context of psychotherapy. Thus far, research exploring the influence of therapists’ personality on psychotherapy treatment outcomes has been limited when compared to the vast amount of research investigating the influence of patient’s personality. However, the studies exploring patients’ personality have not controlled for the influence of the therapist which can lead to overestimated effect sizes because individual therapists can vary considerably in their treatment outcomes (i.e., therapist effects). There is still much to learn about the influence of therapists’ and patients’ personality on psychotherapy outcomes. Through the completion of a systematic review and an empirical study this thesis aimed to advance current understandings about the influence of therapists’ and patients’ personality on psychotherapy treatment outcomes. The first part of this thesis describes the findings of a systematic review that included 27 papers. The systematic review explored the influence therapists’ personality traits had directly on psychotherapy treatment outcomes and indirectly through their influence on therapeutic processes (i.e., interactions that alter the relationship between therapists and patients). Results indicated a relationship between therapists’ personality traits and the model of therapy they preferred to deliver to patients. A relationship was also found between therapists’ personality traits and interpersonal skills (i.e., the ability to interact well with others) associated with positive psychotherapy outcomes. Current findings need to be considered with caution due to high heterogeneity between studies. Part two of this thesis depicts an empirical study that used multilevel model analysis on a retrospective dataset to explore the relationship between patient personality disorder and therapist effects (i.e., the effect therapists have on patient outcomes). Unlike previous research the current study did not reveal a significant therapist effect in any multilevel model analysis. Furthermore, patient personality disorder was not a significant predictor of outcome severity for depression and anxiety once baseline severity was accounted for. This suggests baseline severity is a more reliable predictor of poor treatment outcomes than personality disorder traits. Nevertheless, these findings need to be interpreted with caution due to sample size limitations. The findings of the systematic review and empirical study are discussed in the context of previous literature. Considerations of the clinical implications and recommendations for future research are also discussed.