With recent political developments sparking sharp divisions within democracies, an understanding of the dynamics of polarization is ever more necessary. Yet we still lack the tools necessary for its comparative study at the mass level. Finding that conventional measures of polarization as ideological distance between parties or among voters do not fully capture political polarization, we develop a new index of mass partisan polarization based on support and rejection of political parties by the public. We argue that measuring polarization over political parties allows us to capture divisions over a broader range of identities or issues which parties can represent or take positions on. Using Comparative Study of Electoral Systems data, our empirical validity tests support this argument. It is our hope that this index may facilitate the further comparative study of mass political polarization on a global scale.

Contact between people from different groups triggers specific individual- and group-level responses, ranging from attitudes and emotions to welfare and health outcomes. Standard social psychological perspectives do not yet provide an integrated, causal model of these phenomena. As an alternative, we describe a coalitional perspective. Human psychology includes evolved cognitive systems designed to garner support from other individuals, organize and maintain alliances, and measure potential support from group members. Relations between alliances are strongly influenced by threat detection mechanisms, which are sensitive to cues that express that one's own group will provide less support or that other groups are dangerous. Repeated perceptions of such threat cues can lead to chronic stress. The model provides a parsimonious explanation for many individual-level effects of intergroup relations and group-level disparities in health and well-being. This perspective suggests new research directions aimed at understanding the psychological processes involved in intergroup relations.

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