We predict that people with different political orientations will exhibit systematically different levels of political homophily, the tendency to associate with others similar to oneself in political ideology. Research on personality differences across the political spectrum finds that both more conservative and more politically extreme individuals tend to exhibit greater orientations towards cognitive stability, clarity, and familiarity. We reason that such a “preference for certainty” may make these individuals more inclined to seek out the company of those who reaffirm, rather than challenge, their views. Since survey studies of political homophily face well-documented methodological challenges, we instead test this proposition on a large sample of politically engaged users of the social-networking platform Twitter, whose ideologies we infer from the politicians and policy nonprofits they follow. As predicted, we find that both more extreme and more conservative individuals tend to be more homophilous than more liberal and more moderate ones.