• investigator allegiance;
  • experimenter bias;
  • meta-analysis

The meta-analysis of Luborsky et al. (this issue) confirms the significance of an investigator allegiance effect in psychotherapy research and clarifies the magnitude of this effect. In this commentary, I argue that the investigator allegiance is most likely a proxy measure of the expertise of the investigator's team with the novel therapy and the disproportionately lower credibility and integrity of the comparator condition. Hence, larger allegiance effects are more likely early in the developmental history of research with newer therapies, that is, when pseudotherapies or nonspecific comparators are commonly used. Studies that offer active comparators that are both credible and rigorously and professionally administered are unlikely to show an allegiance effect. Use of “blinded,” independent evaluations of outcome provides another safeguard. The investigator allegiance effect does not need to be an anathema to psychotherapy researchers.