This paper critically considers ‘additional’ and ‘instrumental’ explanations that economists have recently suggested in order to reduce the understanding of CSR within the limits of standard economic theorizing, and contrasts them with a ‘constitutive’ definition as an extended model of corporate governance. Then critically reviews the new institutional economics literature that, although it may not be normally related to CSR, is much more useful than standard microeconomic theorizing for the purposes of gaining deep understanding of CSR. Therefore it is suggested that economists should draw from it in order to understand why fiduciary duties must be expanded even for mere economic reasons of efficiency, and also why corporate reputation cannot leave aside the explicit settlement of an ethical norm of CSR. On these basis, the paper presents a full-fledged social contract theory of multi-stakeholder corporate governance, which entails multiple fiduciary duties and a fair distribution of corporate surpluses and provide and offer an explanation of CSR as a self-enforceable social norm based on the Binmore-Rawls theory of the social contract that underpins multi-stakeholder institutions of corporate governance. Then the paper replies to the main criticisms leveled against the stakeholder model of corporate governance, while at the same time developing analytically the model’s foundation.