In international politics "friends'' co-ally. But friendship is relational and contextual. Countries are more likely to act on common interests on a given dimension if few other actors share that identity. In contrast, new cleavages are likely to emerge as an identity becomes ubiquitous.
The tendency for states to form common alliances based on certain affinities is thus best thought of as a (strategic) variable, rather than as a constant. For example, in systems where democracies are scarce, democratic states tend to seek out democratic allies. As democracy becomes more common, however, incentives binding democratic allies together weakens, eventually giving way to other definitions of mutual interest. The argument, and the evidence we provide here, suggest that national identities are activated by strategic concerns as well as other factors. The salience of identities as cues to affinity and difference vary with the distribution of types in the system.