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I present a model of affective polarization—growth in hostility over time between two parties—via quasi-Bayesian inference. In the model, two agents repeatedly choose actions. Each choice is based on a balance of concerns for private interests and the social good. More weight is put on private interests when an agent's character is intrinsically more self-serving and when the other agent is believed to be more self-serving. Each agent Bayesian updates about the other's character, and dislikes the other more when she is perceived as more self-serving. I characterize the effects on growth in dislike of three biases: a prior bias against the other agent's character, the false consensus bias, and limited strategic thinking. Prior bias against the other's character remains constant or declines over time, and actions do not diverge. The other two biases cause actions to become more extreme over time and repeatedly be “worse” than expected, causing mutual growth in dislike, that is, affective polarization. The magnitude of dislike can become arbitrarily large—even when both players are arbitrarily “good” (unselfish). The results imply that seemingly irrelevant cognitive biases can be an important cause of the devolution of relationships, in politics and beyond, and that subtlety and unawareness of bias can be key factors driving the degree of polarization.