Relational similarity: An introduction and an application to military alliances

The ability of military alliances to deter depends on their credibility: the degree to which others believe that allies will fulfill their commitments. One way to measure credibility is to compare nations' lists of allies. The more similar those lists are, the more similar are those nations' security interests. This increases the credibility and the deterrent capability of resulting alliances and, consequently, decreases the amount of militarized conflict. In order to measure the similarity of alliance lists, one needs to account for the possibilities that countries can have multiple allies, can be indirectly linked to one another by their allies, and can be part of mutually exclusive groups of directly and indirectly related allies. Using a new measure called relational similarity, this article finds support for the credibility argument and also finds that relational similarity better explains observed patterns of conflict than existing measures.