Rationalizable implementation

We consider the implementation of social choice functions under complete information in rationalizable strategies. A strict version of the monotonicity condition introduced by Maskin is necessary under the solution concept of rationalizability. Assuming the social choice function is responsive, i.e. in distinct states it selects distinct outcomes, we show that strict Maskin monotonicity is also sufficient under a mild “no worst alternative” condition. In particular, no economic condition is required. We discuss how our results extend when the social choice function is not responsive.