title: Prediction of placebo responses: a systematic review of the literature.
abstract: OBJECTIVE
  Predicting who responds to placebo treatment-and under which circumstances-has been a question of interest and investigation for generations. However, the literature is disparate and inconclusive. This review aims to identify publications that provide high quality data on the topic of placebo response (PR) prediction.
  
  
  METHODS
  To identify studies concerned with PR prediction, independent searches were performed in an expert database (for all symptom modalities) and in PubMed (for pain only). Articles were selected when (a) they assessed putative predictors prior to placebo treatment and (b) an adequate control group was included when the associations of predictors and PRs were analyzed.
  
  
  RESULTS
  Twenty studies were identified, most with pain as dependent variable. Most predictors of PRs were psychological constructs related to actions, expected outcomes and the emotional valence attached to these events (goal-seeking, self-efficacy/-esteem, locus of control, optimism). Other predictors involved behavioral control (desire for control, eating restraint), personality variables (fun seeking, sensation seeking, neuroticism), or biological markers (sex, a single nucleotide polymorphism related to dopamine metabolism). Finally, suggestibility and beliefs in expectation biases, body consciousness, and baseline symptom severity were found to be predictive.
  
  
  CONCLUSIONS
  While results are heterogeneous, some congruence of predictors can be identified. PRs mainly appear to be moderated by expectations of how the symptom might change after treatment, or expectations of how symptom repetition can be coped with. It is suggested to include the listed constructs in future research. Furthermore, a closer look at variables moderating symptom change in control groups seems warranted.
authors.affiliation: Department of Internal Medicine - Psychosomatic Medicine and Psychotherapy, University Hospital Tübingen Tübingen, Germany ; Department of Psychology, Clemson University Clemson, SC, USA.
authors.forename: Bjoern
authors.initials: B
authors.lastname: Horing
authors.affiliation: Department of Internal Medicine - Psychosomatic Medicine and Psychotherapy, University Hospital Tübingen Tübingen, Germany.
authors.forename: Katja
authors.initials: K
authors.lastname: Weimer
authors.affiliation: Department of Psychology, Clemson University Clemson, SC, USA.
authors.forename: Eric R
authors.initials: ER
authors.lastname: Muth
authors.affiliation: Department of Internal Medicine - Psychosomatic Medicine and Psychotherapy, University Hospital Tübingen Tübingen, Germany.
authors.forename: Paul
authors.initials: P
authors.lastname: Enck
keywords: optimism; pain; personality; placebo; placebo response prediction; self-efficacy