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.We draw on research on personality differences across the political spectrum to develop and test the prediction that people with different political orientations will exhibit different levels of political homophily, the tendency to choose to associate with others similar to oneself in political ideology. Ideological groups with greater political homophily possess political networks with more ties among their members and fewer ties with individuals possessing different ideologies. Thus, greater political homophily is associated with decreased chances of politically diverse interactions and increased rates of interactions with ideologically similar others that tend to reinforce individuals' views and enhance their commitment to their ideological group. These outcomes are in turn likely to increase the polarization of public opinion and promote participation in political collective action.Since at least John Stuart Mill (1859), political theorists have argued that dialogue across lines of political difference is a key pre-requisite for sustaining a democratic citizenry. Mill held that political disagreement enables individuals to develop skills for critically assessing political claims and provides the challenge necessary for determining if one's own ideas are justified. Hannah Arendt similarly argued that debate "constitutes the very essence of political life" (Arendt, 1961, p. 241), irreplaceable for forming enlightened political opinions that reach beyond the limits of one's own subjectivity to 1 0162-895X