Do voters update their perceptions of party positions? How certain do voters feel about their perceptions? In this article for LSE British Politics and Policy, Jae-Hee Jung, Postdoctoral Fellow at the Blavatnik School, and Zeynep Somer-Topcu, Associate Professor at the University of Texas, argue that perceptual updating and perceptual certainty depend on how divided or unified political parties are in their policy positions. They test their arguments in the context of Brexit.