Do Birds of a Feather Flock Together? Rebel Constituencies and Civil War Alliances

Challenging influential perspectives that downplay the role of shared rebel constituencies, we argue that they represent important causes of rebel alliances. Yet, we theorize distinct effects for different types of constituency. While compatible political aspirations push both organizations with a common ideological constituency and those with a common ethnic constituency to ally, for co-ethnic organizations this cooperation-inducing effect is offset by a cooperation-suppressing effect due to their higher risk of inter-rebel war. Leveraging a novel dataset of alliances in multiparty civil wars (1946-2015), we find support for our theoretical expectations. Shared ideological constituencies have a larger and more robust positive effect on the probability of alliances than shared ethnic constituencies. Furthermore, we find that co-ethnic rebel organizations tend to establish informal alliances only, while organizations sharing an ideological constituency are drawn to formal alliances.