Star ratings play major roles in informing consumer decisions on major e-commerce sites such as Amazon.com. When people evaluate political books, for example, they might gauge whether star rating favorability scores align with their pre-existing political attitudes. The attitudinal stance of these online cues could invariably trigger responses that worsen the affective divide between partisans. However, few studies have identified and explained the mechanisms underlying affective polarization. Using two online experiments, we examine the mechanisms through which ratings induce affective polarization. Findings support warranting theory by consistently showing that counter-attitudinal ratings indirectly drive up affective polarization by increasing positive in-party affect via perceived manipulation. By contrast, pro-attitudinal star ratings polarize by increasing positive in-party affect through perceived in-party prototypicality. These findings challenge assumptions that affective polarization is solely characterized by negative out-party affect, and driven mainly by exposure to counter-attitudinal information.