Adolphe Quetelet was a Belgian polymath who aimed to advance aggregate-level statistical tools as a unifying framework for all scientific disciplines. In doing so, Quetelet adopted the astronomer's Law of Error (i.e., the normal distribution curve) and applied it to the study of moral and social phenomena in developing his notion of physique sociale (social physics). Quetelet further focused his attention on l'homme moyen (the average man) and, as such, argued that the average value of a distribution should be of primary concern in the study of human attributes. In the present article, I examine the influences that these ideas had on the methodological practices of late 19th- and early 20th- century psychologists. I illustrate how the dominant methodological approach implemented by psychologists in the early 20th century was deeply rooted in the demography of Quetelet's social statistics. In particular, I argue that psychologists' adoption of the Neo-Galtonian model of research was successful because it embraced Quetelet's determinism, emphasis on average values, and grouping of distributions based on type. (PsycInfo Database Record (c) 2022 APA, all rights reserved).