In health promotion research, enthusiasm for patient and public involvement (PPI) is growing. However, a lack of conceptual clarity leads to ambiguities in participatory processes and purposes, and hampers efforts to achieve and evaluate PPI in research. This study provides an overview of its underlying reasons—or rationales—so as to better understand, guide and interpret PPI in research practice. We conducted a critical review to identify typologies of rationales for PPI. We re-categorized the different types of rationales from these typologies based on their content. We illustrated the resulting categories of rationales with examples from a case study on PPI in research on Lyme disease. Five categories of rationales for PPI were identified. The democratic rationale reflects the normative right of citizens to have a voice in research. The consumerist rationale refers to the economic right of stakeholders with interests to have a say. Rooted in social justice, the transformative rationale seeks to empower marginalized groups. The substantive rationale starts from epistemic considerations and aims to improve the quality of knowledge that research generates. The instrumental rationale is of pragmatic origin and refers to improved efficiency and effectiveness of the research. Our overview of categories of rationales can be used as a frame of reference for PPI in health promotion research. Exploring, stating explicitly and reflecting on the underlying reasons for PPI may help to define realistic purposes, select matching approaches and design appropriate evaluation studies. This might also contribute to the conceptualization of PPI.

Enthusiasm for patient and public involvement in health promotion research is growing. However, it often remains unclear why this involvement is being organized. This lack of clarity makes it difficult to implement such participatory research and to evaluate its added value. We searched for reasons that were given for starting patient and public involvement in research. We found five different reasons. The first is the right of all citizens to have a say in research that affects them. The second is the right of individuals to demand the best research. The third is the desire to involve and empower marginalized groups. The fourth aims to improve the quality of the knowledge generated by the research. The fifth is to achieve more effective and efficient research. Together, these reasons may help researchers get more clarity about why patients or the public should be involved in their studies. This may in turn help to define realistic purposes, design a good participatory process, and conduct appropriate evaluations. All in all, using these five reasons as a frame of reference might lead to a better understanding of what good participatory research in health promotion should look like.

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