The structure of strategy-proof rules

We establish that all strategy-proof social choice rules in strict preference domains follow necessarily a two-step procedure. In the first step, agents are asked to reveal some specific information about their preferences. Afterwards, a subrule that is dictatorial or strategy-proof of range 2 must be applied, and the selected subrule may differ depending on the answers of the first step. As a consequence, the strategy-proof rules that have been identified in the literature for some domains can be reinterpreted in terms of our procedure and, more importantly, this procedure serves as a guide for determining the structure of the strategy-proof rules in domains that have not been explored yet.