Expectancy/credibility change as a mediator of cognitive behavioral therapy for generalized anxiety disorder: Mechanism of action or proxy for symptom change?

Examined the mediating effect of changes in expectancy/credibility from sessions 4-7 of 14-session cognitive and behavioral therapy for generalized anxiety disorder (GAD). In 76 adults with primary GAD, we predicted that expectancy/ credibility would change significantly from sessions 4-7, that degree of change in expectancy/credibility would predict degree of reliable change at posttreatment, and that changes in expectancy/credibility would mediate the relationship between pretreatment severity and change at posttreatment. In support of the hypotheses, a latent growth model revealed significant increases in expectancy/credibility over the critical period. In addition, baseline GAD severity, expectancy/credibility intercept, and rate of change in expectancy/credibility all positively predicted degree of reliable change at posttreatment. Rate of change in expectancy/credibility during the critical period partially mediated the effect of baseline GAD severity, accounting for 38% of the variance in this relationship. This effect was not accounted for by preceding or concurrently changing anxiety levels.