At its inception, experimental psychology was devoted to the study of single subjects in the quest for knowledge of what is true “in general,” in the sense of being common to all of the investigated individuals. Gradually, however, the discipline abandoned the single-subject approach in favor of a treatment group approach. This latter approach is reliant on populationlevel statistical methods and is suited to the quest for knowledge of what holds for individuals “in general” only in the sense of true on average. In the present article, I argue that this epistemically radical change in investigative methods effectively transformed the discipline, in a way that has remained largely unrecognized, from a genuine psychology into a species of demography, that is, from a discipline formally suited to advancing our knowledge about the psychological functioning of individuals into one suited only to the production of knowledge about populations. The implications of this transformation for the discipline moving forward are briefly considered.

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