After society has decided on a social choice rule-a recipe for choosing the optimal social alternative (or alternatives) on the basis of individuals' preferences over the set of all social alternatives-the social planner still faces the problem of how to implement that rule. In particular, the planner may not know individuals' preferences. He might attempt to elicit them, but this may not be an easy task, even abstracting from communication costs. If individuals know the rule by which the planner selects alternatives on the basis of reported preferences, they may have an incentive to report falsely.

Eric Maskin, "Nash equilibrium and welfare optimality." The Review of Economic Studies 66.1 (1999): 23-38.

https://web.archive.org/web/20160305074435/http://...