How do people go about reading a room or taking the temperature of a crowd? When people are briefly exposed to an array of faces, they can only sample a subset of them. We propose that perceivers preferentially attend to more emotional faces and that this generates a crowd emotion amplification effect—estimating a crowd’s average emotional response as more extreme than it is. Study 1 (N = 50) documents the crowd amplification effect. Study 2 (N = 50) replicates the effect even when we increase exposure time. Study 3 (N = 50) uses eye-tracking to show that attentional bias to emotional faces drives amplification. These findings have important implications for every domain in which individuals have to make snap judgments regarding a crowd’s emotionality, from public speaking to controlling crowds.