Individuals who violate expectations increase uncertainty during social interactions. Three experiments explored whether expectancy-violating partners engender "threat" responses in perceivers. Participants interacted with confederates who violated or confirmed expectations while multiple measures were assessed, including cardiovascular reactivity, task performance, appraisals, and behavior. In Experiments 1 and 2, participants interacted with White or Latino confederates who described their family backgrounds as either high or low socioeconomic status. In Experiment 3, participants interacted with Asian or White confederates who spoke with expected accents or southern accents. Participants interacting with expectancy-violating partners (e.g., Asians with southern accents) exhibited cardiovascular responses consistent with threat, poorer task performance, and manifested negative and defeat-related behavior. Implications for decreasing prejudicial responses via uncertainty reduction are discussed.Keywords: uncertainty, expectancy violations, cardiovascular reactivity, challenge and threat Interacting with strangers can be stressful. During such interactions, individuals are simultaneously attending to their own thoughts, feelings, and behaviors, including self-presentational, self-regulatory, and impression management concerns, as well as trying to accurately perceive the characteristics of the person to establish conversational fluency, expectations, and social norms (DePaulo & Friedman, 1998;Gilbert, 1998;Gudykunst, 1984). To the extent that the stranger meets expectations, the interaction is likely to be routinized and predictable. However, when the stranger is unexpected or surprising in some way, routinized responses are likely to be disrupted and anxiety heightened (Hebl, Tickle, & Heatherton, 2000). In the present study, we tested the notion that expectancy violations engendered by interaction partners would create more situational uncertainty and increase psychological threat as marked cardiovascularly, affectively, and behaviorally.We have previously observed that, in general, African Americans relative to European Americans engendered threat responses among White participants during a pleasant social interaction. More important, the degree of past intergroup contact with African Americans reported by White participants moderated responses, such that the greater the contact with African Americans the lower the threat responses. This finding underscores the importance of contact during social interactions. But what about situations in which one could not have experienced contact because the stranger is surprising or unexpected in some way, that is, a stranger who violates expectancies?Expectancies function to help people predict the future on the basis of past experiences and knowledge (Olson, Roese, & Zanna, 1996). Hence, expectancy violations disrupt one's predictive ability and can create uncertainty. This uncertainty is likely to increase the demands of situations, diverting attentional resources away...