How does democratic politics inform the interdisciplinary debate on the evolution of human co-operation and the social preferences (for example, trust, altruism and reciprocity) that support it? This article advances a theory of partisan trust discrimination in electoral democracies based on social identity, cognitive heuristics and interparty competition. Evidence from behavioral experiments in eight democracies show ‘trust gaps’ between co- and rival partisans are ubiquitous, and larger than trust gaps based on the social identities that undergird the party system. A natural experiment found that partisan trust gaps in the United States disappeared immediately following the killing of Osama bin Laden. But observational data indicate that partisan trust gaps track with perceptions of party polarization in all eight cases. Finally, the effects of partisanship on trust outstrip minimal group treatments, yet minimal-group effects are on par with the effects of most treatments for ascriptive characteristics in the literature. In sum, these findings suggest political competition dramatically shapes the salience of partisanship in interpersonal trust, the foundation of co-operation.