This paper builds on advances in social ontology to develop a new understanding of how mainstream economic modelling affects reality. We propose a new framework for analyzing and describing how models intervene in the social sphere. This framework allows us to identify and articulate three key epistemic features of models as interventions: specificity, portability, and formal precision. The second part of the paper uses our framework to demonstrate how specificity, portability and formal precision explain the use of moral hazard models in a variety of different policy contexts including worker compensation schemes, bank regulation and the euro-sovereign debt crisis.