Equilibrium play in voluntary ultimatum games: Beneficence cannot be extorted

An exceptionally robust result in experimental economics is the failure to observe equilibrium (subgame perfect) play in the ultimatum game. A heretofore unnoticed feature of the game is that neither player voluntarily chooses to play. Motivated by Adam Smith's proposition that beneficence—like that of non-equilibrium play in the ultimatum game—cannot be extorted by force, we offer the responder the opportunity to opt out of the game for a mere $1 payoff for both players. We observe far higher rates of equilibrium play, including highly unequal splits, than heretofore reported in binary choice versions of the game.