The value of lies in an ultimatum game with imperfect information

Humans often lie strategically. We study this problem in an ultimatum game with an informed proposer and an uninformed responder, where the former can send an unverifiable statement about his endowment. A simple message game with heterogenous players with respect to lying costs shows that in equilibrium liars should understate their endowment. The second part of the paper reports on an experiment testing this game. On average, 88.5% of the proposers understate the actual endowment by 20.5%. Regression analysis shows that a 1-euro gap between the actual and declared amounts prompts proposers to reduce their offer by 19 cents on average. However, responders' decision to accept/reject the offer does not depend on the message. It results a net welfare loss specific to such a “free-to-lie” environment.