Intervention Effectiveness of The Incredible Years: New Insights Into Sociodemographic and Intervention-Based Moderators

•BPT effectiveness may be moderated by sociodemographic and intervention factors.•Moderators of BPT have been mostly tested in separate studies and small samples.•Previously suggested moderators were tested in a multi-informant, large-scale RCT.•BPT was effective in decreasing parent-reported child externalizing behavior.•No systematic evidence for the suggested moderators was found.

We tested the effectiveness of the preventive behavioral parent training (BPT) program, The Incredible Years (IY), and the independent effects of previously suggested sociodemographic and intervention-based moderator variables (i.e., initial severity of externalizing problem behavior, child gender, social economic status, family composition, and number of sessions parents attended), in a large-scale randomized controlled trial. Questionnaire and observation data from 387 parents and children ages 4-8 years (Mage= 6.21, SD = 1.33, 55.30% boys) across pretest, posttest, and 4-month follow-up were analyzed, using full intention-to-treat analyses and correcting for multiple testing. IY was successful in decreasing parent-reported child externalizing behavior (Cohen's d = 0.20 at posttest, d = 0.08 at follow-up), increasing parent-reported (d = 0.49, d = 0.45) and observed (d = 0.06, d = 0.02) positive parenting behavior, and decreasing parent-reported negative parenting behavior (d = 0.29, d = 0.25). No intervention effects were found for reported and observed child prosocial behavior, observed child externalizing behavior, and observed negative parenting behavior. Out of 40 tested moderation effects (i.e., 8 Outcomes × 5 Moderators), only three significant moderation effects appeared. Thus, no systematic evidence emerged for moderation of IY effects. The present multi-informant trial demonstrated that many previously suggested moderators might not be as potent in differentiating BPT effects as once thought.