Utilitarianism has frequently been criticized for lacking psychological realism, but what this means and why it is thought to matter varies. This article distinguishes and examines three main relevant kinds of appeals to psychological realism: (a) A minimalist, self-avowedly metaethically neutral and empirically based ‘ought implies can’ approach, exemplified by Owen Flanagan. (b) Arguments from psychological costs and flourishing, exemplified by Michael Stocker and Bernard Williams. (c) ‘Thick’ psychological realism, exemplified by Elizabeth Anscombe, where a conception of human nature does not simply provide constraints on value theory, but forms the substantive basis on which it builds. The main challenge raised for utilitarianism turns out to be metaethical, not a matter of empirical psychology. The question is not so much whether utilitarianism can accommodate (putative) descriptive facts of human psychology as such, but what normative weight these facts should be given and why.