Regardless of whether Americans actually hold more extreme policy preferences, they have responded to a polarized political climate by adopting stronger negative attitudes about each other. How? The consequence of END interactions are an important part of the explanation. We first review the affective origins of American political identities, the social divisions that are known to map onto political identities, the prevalence of psychological polarization, and the structural forces and social psychological explanations that have encouraged its development. We then preview the book’s central argument, unpacking the steps linking Facebook usage with psychological polarization. Each step reflects the integration of the hallmark distinctions of END communication: the context of byproduct exposure and social inference, the signal rich content, and the disproportionate influence of weak ties in the constitution of the interactions. We outline a series of testable implications from the END Framework about the effects of frequent Facebook usage and increased exposure to politically informative News Feed content on 1) users’ ability and confidence to draw inferences about the political identities of their connections on the site, 2) the biases in their reasoning about the political preferences of their connections, and 3) the judgments they make about their in-group and out-group connections.