Since its introduction in 1943, the term “problem families” has been used to classify a relatively small sector of society assumed to cause problems disproportionate to its size. This paper examines a project conducted by the British Eugenics Society that attempted to address this very issue, culminating with the Problem Families Committee (1947-1952). A key aspect of this study involves exploring the interaction between eugenic ideology and the porous border that loosely segregated “nature” and “nurture” interpretations of human society. With this in mind, I examine: first, the rise of the term “mental deficiency” and its attempted use by eugenicists to explain the existence of the poor in the inter-war period; second, the emergence of “Problem Families” as a typological sub-section of society, in some respects considered hereditarily degenerative but, significantly, also a product of the inequalities of modern, urban society; and finally, I draw tentative conclusions on why the term “problem families” still resonates in popular and political discussion today. I operate here on the premise that the “problem families” discourse was characterised both by a ubiquitous struggle to define the “problem family” and a wider metaphysical struggle concerning the nature of heredity and what it meant to be human.