The term “underdog effect” denotes a phenomenon of public opinion impinging upon itself: When at an election or referendum or with regard to opinions on issues citizens perceive a particular party, candidate, or political position to be the likely winner, or at least to be dominant, they support an alternative that is expected to lose. It has most intensely been discussed with regard to elections, and usually alongside the corresponding phenomenon of bandwagon effects. Supportive empirical evidence is rather weak. A plausible theoretical interpretation has been developed only recently. The utility of the concept for more complex choice situations, especially multiparty systems, appears doubtful.