Aims: Motivational interviewing (MI) is an efficacious treatment for substance use disorders. However, little is known about how MI exerts its therapeutic effects. This review is a first attempt to summarize and evaluate the evidence for purported within-session mechanisms of change. The primary question of interest was: which MI constructs and variables appear to be the most promising candidates for mechanisms of change?
Methods: Literature searches were conducted to identify studies delivering MI in an individual format for the treatment of substance use disorders. Our search identified a total of 152 studies for review; 19 studies met inclusion criteria by providing data on at least one link in the causal chain model under examination. Effect size estimates were calculated for every possible step in the causal model where sufficient data were provided by study authors.
Results: Four constructs of therapist behavior were evaluated: MI-Spirit, MI-Consistent behaviors, MI-Inconsistent behaviors and therapist use of specific techniques. Five constructs of client behavior were evaluated: change talk/intention, readiness to change, involvement/engagement, resistance and the client's experience of discrepancy. The absence of experimental and full mediation studies of mechanisms of change was notable. Effect sizes were generally mixed.
Conclusions: The most consistent evidence was found for three constructs: client change talk/intention (related to better outcomes); client experience of discrepancy (related to better outcomes); and therapist MI-Inconsistent behavior (related to worse outcomes). Regarding therapist use of specific techniques, use of a decisional balance exercise showed the strongest association to better outcomes.