Empirical support for naturalistic developmental behavioral interventions (NDBIs) is growing, suggesting that this class of early interventions are effective for supporting social communication development in young autistic children. NDBIs are frequently implemented as parent mediated interventions, in which caregivers are taught specific treatment techniques to implement with their child. Parent mediated NDBIs are complex, involving two levels of service delivery (coach to parent, and parent to child) each comprised of multiple intervention elements. Despite their promise, little research has examined the active ingredients and mechanisms of change underlying NDBIs, and studies have yet to consider unique features of parent-mediated approaches that may affect how they work. The present study used an exploratory sequential mixed methods design grounded in the theory of change framework to explore the active ingredients and mechanisms of change of Project ImPACT, an empirically supported NDBI for young children with autism or social communication delays. The first study aim was to develop a comprehensive Theory of Change by obtaining stakeholder perspectives on how Project ImPACT works. Ten intervention experts, 22 community providers, and 12 caregivers participated in semi structured interviews which were subsequently coded using qualitative methods. We used joint displays to develop and visualize causal models for each intervention element, which we then consolidated in a broader visual model of intervention process at the parent and child level. The second aim was to provide proof-of-concept of our model using quantitative analyses conducted with archival data from treatment trials of Project ImPACT. Through completion of aim 1, we developed two integrated models of the change process for Project ImPACT, with the first detailing the caregiver learning process as the caregiver works with a coach, and the second detailing the child learning process as the caregiver implements the intervention techniques. The caregiver learning model emphasized the importance of both learning and motivational process in supporting fidelity and sustainment. The child learning model described how developmental techniques aimed at supporting children’s attention and engagement lay the foundation for more adult-directed learning opportunities and subsequent child skill growth. However, aim 2 hypotheses were not supported by path analyses conducted using archival data. Our qualitative data also described key themes relating to long-term outcomes (e.g., sustainment and quality of life for children and families), need for a service like Project ImPACT (e.g., structured and systematic, child-centered, parent-driven service), and contextual factors that affect fit (e.g., child- and family-level characteristics, cultural and linguistic factors). Developing, refining, and testing program theory is a core element of research in complex interventions, with several benefits for research and implementation in practice. Although parent mediated NDBIs are increasingly being studied in research contexts, their active ingredients and mechanisms of change are understudied, and their complexity presents a barrier to widespread implementation in the community. In the future, this research can be used to develop and prioritize nuanced research questions related to the timing, optimization, and mechanistic process underlying NDBIs.
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