The thesis of methodological individualism in social science is commonly divided into two different claims—explanatory individualism and ontological individualism. Ontological individualism is the thesis that facts about individuals exhaustively determine social facts. Initially taken to be a claim about the identity of groups with sets of individuals or their properties, ontological individualism has more recently been understood as a global supervenience claim. While explanatory individualism has remained controversial, ontological individualism thus understood is almost universally accepted. In this paper I argue (...) that ontological individualism is false. Only if the thesis is weakened to the point that it is equivalent to physicalism can it be true, but then it fails to be a thesis about the determination of social facts by facts about individual persons. Even when individualistic facts are expanded to include people’s local environments and practices, I shall argue, those still underdetermine the social facts that obtain. If true, this has implications for explanation as well as ontology. I first consider arguments against the local supervenience of social facts on facts about individuals, correcting some flaws in existing arguments and affirming that local supervenience fails for a broad set of social properties. I subsequently apply a similar approach to defeat a particularly weak form of global supervenience, and consider potential responses. Finally, I explore why it is that people have taken ontological individualism to be true. (shrink)