Indiana

They discussed this on NPR's On The Media http://www.onthemedia.org/

At least one academic study has shown the tactic to be effective. Researchers from Yale University and the University of Northern Iowa performed an experiment in Michigan in the 2000s that showed that the most effective way to boost turnout through a mailed reminder was to include the voting participation histories of the recipient and his or her neighbors. That was four times as effective as sending a simple reminder that voting is a duty.