How do people go about reading a room or taking the temperature of a crowd? When people catch a brief glimpse of an array of faces, they can only focus their attention on some of the faces. We propose that perceivers preferentially attend to faces exhibiting strong emotions, 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 many domains in which individuals have to make snap judgments regarding a crowd’s emotionality, from public speaking to controlling crowds.