American partisans are far more hostile towards out-party members than they were 40 years ago. While this phenomenon, often called affective polarization, is well-documented, political scientists disagree on its cause. One group of scholars believes that affective polarization is driven by processes related to social identity theory. In particular, cross-cutting identities have declined in America, and toxic political communication continuously primes partisan identities and resentment. Recently, several scholars have pointed to another phenomenon as the root cause of affective polarization: partisan sorting, i.e. the alignment of partisan identities with ideologically consistent issue positions. I review evidence in favor of each claim, and provide additional evidence that affective polarization has increased about as much among those who are not sorted as among those who are sorted. Furthermore, while sorting is only related to affective polarization among the most politically knowledgeable, affective polarization has increased across all levels of political knowledge. Finally, affective polarization may also increase sorting, further complicating any clear cut causal relationship.