W . ALLEN WALLIS Institute of POLITICAL ECONOMY UNIVERSITY OF ROCHESTER

This paper analyzes collective choices in a society with strategic voters and single-crossing preferences. It shows that, in addition to single-peakedness, single-crossingness is another meaningful domain which guarantees the existence of non-manipulable social choice functions. A social choice function is shown to be anonymous, unanimous and strategy-proof on single-crossing domains if and only if it is an extended median rule with n − 1 parameters distributed on the end points of the feasible set of alternatives. Such rules are known as positional dictators, and they include the median choice rule as a particular case. As a by-product, the paper also provides an strategic foundation for the so called “single-crossing version” of the Median Voter Theorem, by showing that the median ideal point can be implemented in dominant strategies through a simple mechanism in which each agent honestly reveals his preferences. JEL codes: D70, D71.