An allegiance to a school of psychotherapy may distort the findings of treatment effectiveness in randomized clinical trials (RCT).

To assess the influence of allegiance in treatment effectiveness.

Using a systematic approach across various psychotherapy studies we aim to identify if allegiance introduces systematic bias in the observed effects.

We considered meta-analyses of RCT of different types of psychotherapies in the Cochrane Database. Eligible articles included were those that had only RCTs and at least 1 study with allegiance of the experimenter. For each metaanalysis we calculated the relative odds ratio (ROR) of the allegiance studies vs. no allegiance odds ratios per meta-analysis and the summary ROR across all meta-analyses by using random effects models. Heterogeneity is quantified with the I2 metric.

A total of 30 meta-analyses including 240 RCTs were analyzed. Effect sizes of experimental intervention with psychotherapy allegiance (PA) has more favorable effect than experimental intervention without PA (sROR=1.31, 95% confidence interval (CI) 1.03-1.66, I2=53%). Subgroup analyses showed that this inflation is strongly type of psychotherapydependent (Cognitive Behavioral Therapy: sROR=1.07, 95% CI: 0.85-1.34, I2=19%; Supportive or counseling therapy: sROR=1.44, 95% CI: 1.01-2.05, I2=0%; and others: sROR=2.34, 95% CI: 1.03-1.66, I2=79%). Allegiance effect was also stronger where the therapeutic experimenter had both developed the therapy and supervised the therapist implemented the intervention (sROR=2.39 95% CI=1.15 - 4.99 I2=65).

PA can introduce systematic bias on effect sizes of various psychotherapy intervention trials in a similar way that conflict of interest does.