Moderators and mediators of treatment outcome for youth with ADHD: understanding for whom and how interventions work.

OBJECTIVE To present data on moderators and mediators of treatment response from the Multimodal Treatment Study of Children With ADHD (MTA). METHODS Moderator variables (baseline factors that define subgroups with greater vs lesser intervention response) and mediator variables (factors occurring during treatment that explain how interventions "work") are described with specific application to the outcomes of the MTA Study. RESULTS Key moderator variables (comorbid anxiety disorder, public assistance, severity of attention-deficit/hyperactivity disorder, parental depressive symptomatology, IQ) and mediator processes (negative/ineffective parental discipline) are reviewed. CONCLUSIONS Treatment research in the future should explicitly consider the exploration of moderator and mediator variables, which can greatly aid the explanatory power of clinical trials and specify the critical next steps for intervention research.