Research suggests that the mind contains a set of adaptations for detecting alliances: an alliance detection system, which monitors for, encodes, and stores alliance information and then modifies the activation of stored alliance categories according to how likely they will predict behavior within a particular social interaction. Previous studies have established the activation of this system when exposed to explicit competition or cooperation between individuals. In the current studies we examine if shared political opinions produce these same effects. In particular, (...) if participants will spontaneously categorize individuals according to the parties they support, even when explicit cooperation and antagonism are absent, and if party support is sufficiently powerful to decrease participants’ categorization by an orthogonal but typically-diagnostic alliance cue . Evidence was found for both: Participants spontaneously and implicitly kept track of who supported which party, and when party cross-cut race—such that the race of targets was not predictive of party support—categorization by race was dramatically reduced. To verify that these results reflected the operation of a cognitive system for modifying the activation of alliance categories, and not just socially-relevant categories in general, an identical set of studies was also conducted with in which party was either crossed with sex or age . As predicted, categorization by party occurred to the same degree, and there was no reduction in either categorization by sex or by age. All effects were replicated across two sets of between-subjects conditions. These studies provide the first direct empirical evidence that party politics engages the mind’s systems for detecting alliances and establish two important social categorization phenomena: that categorization by age is, like sex, not affected by alliance information and that political contexts can reduce the degree to which individuals are represented in terms of their race. (shrink)