Mechanisms of child behavior change in parent training: Comment on Weeland et al. (2018).

Recently in this journal, Weeland et al. (2018) published a thought-provoking article reporting moderating effects of children's serotonin transporter-linked polymorphisms (5-HTTLPR) on negative parenting during prevention with the Incredible Years series. Participants were parents and young children of 387 families enrolled in the Observational Randomized Control Trial of Childhood Differential Susceptibility study. An equally important finding, which we focus on in this comment, involved null effects for all tests of parenting as a mediator of prevention-induced improvements in children's externalizing behavior. Although such findings may seem surprising, both confirmations of and failures to confirm parenting change as a mediator of child behavior change are common in the prevention and intervention literatures. In this comment, we explore likely reasons for heterogeneity in findings, including both moderators of treatment effect size and methods used to test mediation. Common moderators of prevention and intervention response to Incredible Years include dose, parenting problems at intake, high-risk versus clinical nature of samples, how parenting is measured, and whether child training is included with parent training. All of these moderators affect power to detect mediation. We then discuss conceptual criteria for testing mediation in randomized clinical trials, and problems with interpreting mediating paths in cross-lag panel models. Although the gene effect reported by Weeland et al. is important, their cross-lag panel models do not provide strong tests of parenting as a mediator of child behavior change. We conclude with recommendations for testing mediation in randomized clinical trials.