Both positive mental health and psychopathology should be monitored in psychotherapy: Confirmation for the dual-factor model in acceptance and commitment therapy

•The effects of ACT on depression/anxiety symptoms and positive mental health are only moderately related to each other.•A large majority of participants improved on either depression/anxiety symptoms or positive mental health, not both.•Both psychopathology and positive mental health need to systematically evaluated in clinical research and interventions.•An average treatment effect does not automatically mean that the therapy is effective for all patients and on all outcomes.

The dual-factor model of mental health suggests that enhancing positive mental health and alleviating psychopathology do not automatically go hand-in-hand. This study investigates the relationship between the effectiveness on depression/anxiety symptoms and positive mental health of Acceptance and Commitment Therapy (ACT). It draws on RCT data (n = 250) of a self-help ACT. Patients' depression/anxiety symptoms and positive mental health were completed at baseline, at post-intervention after nine weeks, and at follow-up after five months. Percentage of unique variance of depression/anxiety symptoms explained by positive mental health (and vice versa), and the degree of classificatory agreement between improvements in positive mental health and depression/anxiety, were examined using regression analysis and Reliable Change Index (RCI). Positive mental health, i.e. baseline and change, explained 15% and 12% of the variance in follow-up depression and anxiety symptoms, beyond the 7% and 9% that was explained by baseline levels of depression and anxiety. Depression and anxiety symptoms, i.e., baseline and change, explained 10% and 9% of the variance in follow-up positive mental health, on top of the 35% that was explained by baseline levels of positive mental health. Cross-classification of the Reliable Changes showed that 64% of the participants that improved during the ACT-intervention, improved on either depression symptoms or positive mental health, and 72% of the participants improved on either anxiety symptoms or positive mental health. The findings support the dual-factor model and suggest that it is important to systematically implement measures of both psychopathology and positive mental health in mental health care and therapy evaluations.