Some international organizations are governed by unanimity rule, some others by a majority system. Still others have moved from one system to the other over time. The existing voting models, which generally assume that decisions made by voting are perfectly enforceable, have a difficult time explaining the observed variation in governance mode, and in particular the widespread occurrence of the unanimity system. We present a model whose main departure from standard voting models is that there is no external enforcement mechanism: each country is sovereign and cannot be forced to follow the collective decision, or in other words, the voting system must be self-enforcing. The model yields unanimity as the optimal system for a wide range of parameters, and delivers rich predictions on the variation in the mode of governance, both across organizations and over time.

Some international organizations are governed by unanimity rule, others by (simple or qualified) majority rules. Standard voting models, which assume that the decisions made by voting are perfectly enforceable, have a hard time explaining the observed variation in governance mode, and in particular the widespread occurrence of the unanimity system. We present a model whose main departure from standard voting models is that the organization cannot rely on external enforcement mechanisms: each country is sovereign and cannot be forced to comply with the collective decision or, in other words, the voting system must be self-enforcing. The model identifies conditions under which the organization adopts the unanimity rule, and yields rich comparative-statics predictions on the determinants of the mode of governance.