What changes when? A reply to Beauchaine and Slep

In their commentary, Beauchaine and Slep (2018) raise important issues regarding research on behavioral parenting training (BPT). In this reply we highlight key points of agreement and respond to issues that we feel require clarification. BPT has been repeatedly proven effective in decreasing disruptive child behavior (also in the work of our research team). Yet, there is much to learn about for whom and how BPT is effective. Specifically, assessing the how (i.e., mediation) comes with many challenges. One of these challenges is taking into account the timeline of change, and being able to infer causal mechanisms of change. We argue that cross-lagged panel models (which we, and many other scholars, used) are a valid and valuable method for testing mediation. At the same time, our results raise important questions, specifically about the timing and form of expected changes in parenting and child behavior after BPT. For example, are these changes linear and gradual or do they happen more suddenly? To select the appropriate design, assessment tools, and statistical models to test mediation, we need to state detailed hypotheses on what changes when. An important next step might be to assess multiple putative mediators on different timescales, not only before and after, but specifically also during BPT.