One plausible conclusion from these data is that parent work cannot enhance effects beyond what is achieved through ICBT alone. This study focuses on an alternative plausible conclusion: That parent interventions need to be informed by theoretical working models of parent-specific mechanisms of change that are not targeted in ICBT; and that parent interventions need to be evaluated in explanatory RCTs that ask not only 'does treatment work?' but also 'how and for whom does treatment work?' Underlying systems that shape how parents respond to child anxiety or distress can provide clues to parent-specific targets for intervention and can point to potential moderators of treatment response. Identifying mechanisms by which parent interventions can enhance child anxiety outcomes, and identifying markers of parents most in need of such interventions advances the goal of personalized psychotherapy, and is the overall goal of this study.