Across six studies, we examine how the magnitude of expressed happiness influences social perception and interpersonal behavior. We find that happiness evokes different judgments when expressed at high levels than when expressed at moderate levels, and that these judgments influence opportunistic behavior. Specifically, people perceive very happy individuals to be more naive than moderately happy individuals. These perceptions reflect the belief that very happy individuals shelter themselves from negative information about the world. As a result of these inferences, very happy people, relative to moderately happy people, are more likely to receive biased advice from advisors with a conflict of interest and are more likely to be chosen as negotiation partners when the opportunity for exploitation is salient. Our findings challenge existing assumptions in organizational behavior and psychology by identifying a significant disadvantage of expressing happiness, and underscore the importance of examining emotional expressions at different magnitudes. We call for future work to explore how the same emotion, experienced or expressed at different levels, influences judgment and behavior. (C) 2016 Published by Elsevier Inc.