Previous research by Huber, Payne, & Puto (1982) and by Tyszka (1983) has demonstrated that adding a 3rd (decoy) alternative that is dominated by only 1 of the other 2 alternatives in a choice set increases the preference for the dominating alternative. By manipulating decoys within-subjects, significant preference reversals have been obtained. The processes of 3 classes of models can account for these reversals: (1) The decoy affects the weights assigned to different dimensions, (2) the decoy produces range-frequency effects on the dimensional values of the alternatives, and (3) the perception of dominance directly increases the attractiveness of the targeted alternative. The results of 3 experiments designed to test these models favored a direct effect of dominance. When a dominated decoy was added to choice set, asymmetrical dominance was demonstrated to be a necessary and sufficient condition for producing the predicted preference reversals.