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. (JEL D72, F53)
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