Collusive agreements are often observed in procurement auctions. They are probably more easily achieved when competitors’ costs are easily estimated. If, however, the individual costs of bidders are private information, effective ring formation is difficult to realize. We compare...

We study an ultimatum experiment in which the responder does not know the offer when accepting or rejecting. Unconditional veto power leads to acceptances, although proposers are significantly greedier than in standard ultimatum games, and this is anticipated by responders.