Negative externalities and equilibrium existence in competitive markets with adverse selection

Rothschild and Stiglitz (1976) show that there need not exist a competitive equilibrium in markets with adverse selection. Building on their framework we demonstrate that externalities between agents - an agent's utility upon accepting a contract depends on the average type attracted by the respective principal - can solve the equilibrium existence problem, even when the size of the externalities is arbitrarily small. Our result highlights the degree of control a principal has over the attractiveness of his contracts as an important feature for equilibrium existence, thereby offering a new perspective on existing theories of competition in markets with adverse selection.