We explore how regret influences strategic interaction and risky choice. Regret is captured by the payoff gap between what a player actually gets and what he believes he would have gotten had he chosen differently. Ex-post beliefs are critical to that evaluation, and the modeling therefore draws on tools from psychological game theory. Our analysis uncovers several non-standard implications. From a technical viewpoint, predictions depend in novel ways on information structure across end-nodes, assumptions regarding the precise nature of chance moves and mixed strategies, and the order in which play proceeds. From an applied viewpoint, regret can have powerful impact in a variety of economic settings including education, delegation, gambling, provision of public goods, and market entry.

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