Person, Population, Mechanism. Three Main Branches of Psychological Science.

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There are different ways of dividing psychology into subdisciplines. The purpose of the present paper is to explore one specific meta-perspective on psychological science, seen as having three main branches: person psychology, population psychology, and mechanism psychology, linked to three different levels of research. Person-level research focuses on psychological phenomena as experienced and enacted by individual persons in their interaction with other persons and other parts of the environment, and in their development over time. Population-level research focuses on populations of individuals, frequencies of various psychological phenomena in a population, risk factors, and population-level effects of various psychological interventions. Mechanism-level research focuses on psychological functioning as explained in terms of neurophysiological mechanisms and information processes at a sub-personal level. It is argued that the failure to differentiate clearly between research questions at these three levels lead to questionable research practices. Most notably, a failure to differentiate clearly between the population level and the person level leads to problem-method mismatches in the form of researchers trying to answer questions about persons by research on populations. Also, because of a failure to differentiate between the person level and the mechanism level, explanations in terms of sub-personal mechanisms are too often seen as providing answers about what occurs at the person level, thereby failing to study persons as intentional agents in interaction with other persons and other parts of the environment. It is argued that a clear differentiation between three levels of psychological science - population, person, and mechanism - may contribute to an increased clarity in these matters and may thereby contribute to the development and maturation of psychological science.