Person, Population, Mechanism. A Rejoinder to Critics and an Elaboration of the Three-Branch Model

In a previous paper (Lundh, 2023), it was argued that psychological science can be seen as having three main branches, corresponding to three levels of research: research at the person level, at the population level, and at the mechanism level. The purpose of the present paper is to discuss the critique that has been raised against this model by Lamiell (2024) and Nilsson (2024) and to elaborate and specify the three-branch model in more detail. This is done by an incorporation of Nilsson's concept of person-sensitivity into the model, and by a clearer differentiation between the two contrasts involved: (1) the methodological focus either on individual persons or on populations of individuals; and (2) the theoretical focus either on whole-person functioning or on sub-personal mechanisms.