This article deals with the idea of the performativity of economics. We use the notion of convention in order to emphasize a necessary condition for performativity of the scientific conventions. We show that performativity could be seen as the translation of a scientific convention into the social world. We demonstrate that such a translation needs the scientific concept to take a peculiar form: an empirical one. We study the example of the performativity of economic rationality, which is now a central concept of a new kind of public policy: “nudge” economics.