The concept of proportionality has been central to the retributive revival in penal theory, and underlies desert theory's normative and practical commitment to limiting punishment. Theories of punishment combining desert-based and consequentialist considerations also appeal to proportionality as a limiting condition. In this paper we argue that these claims are founded on an exaggerated idea of what proportionality can offer, and in particular fail properly to consider the institutional conditions needed to foster robust limits on the state's power to punish. The idea that appeals to proportionality as an abstract ideal can help to limit punishment is, we argue, a chimera: what has been thought of as proportionality is not a naturally existing relationship, but a product of political and social construction, cultural meaning-making, and institution-building. Drawing on evolutionary psychology and comparative political economy, we argue that philosophers and social scientists need to work together to understand how the appeal of the idea of proportionality can best be realised through substantive institutional frameworks under particular conditions.