For the first time in a quarter century, majorities of Democrats and Republicans hold “very unfavorable” views of one another. Nearly half of partisans report that the other party makes them feel afraid and angry, many of whom simply “can’t understand” why someone would disagree with their own views (Pew Research Center, 2016). These are the hallmarks of political polarization, which continues to rise in the US despite the negative interpersonal consequences. The present research investigates whether polarization is caused by the side-effect effect – a bias in how we judge whether outcomes are intentionally or incidentally caused. When people are presented with situations where an outcome is described as a mere byproduct, or side-effect, we expect them to say that the outcome was caused unintentionally. However, when a side-effect is perceived as harmful, people tend to view it as being intentionally brought about (Knobe, 2003). In the present study, we demonstrate an asymmetry in the side-effect effect when people are asked to judge the intentions of Democrats and Republicans. Subjects view the harmful side-effects of the other party’s policies as being intentionally caused, but view the harmful side-effects of their own party’s policies as incidental. Intentionality judgments are associated with more extreme negative attitudes toward members of the opposite party, and this relationship is mediated by the perceived morality of policy side-effects. These findings reveal how polarization is exacerbated by basic cognitive processes, and may ultimately yield insights on how to halt or reverse its course.