Civil war is highly concentrated in poor countries. Nearly all recent quantitative studies find a strong relationship between per capita income and risk of civil war onset, controlling for other explanatory variables. How can we explain this? Four different explanations have reached prominence in the quantitative literature. First, the grievance explanation holds that people in poor countries for various reasons are likely to be more frustrated with their government, making them more willing to rebel. Second, the state weakness explanation holds that poor countries are more prone to civil war because their state organizations are less capable of controlling social relationships in general and defeating rising insurgencies in particular. Third, the greed explanation holds that poor countries have a higher risk of civil war because lower economic opportunities imply a greater incentive for people to become rebels. Finally, what I call the ecological explanation holds that civil wars more often occur in poor countries because they have social and physical environments more conducive to guerilla warfare - the usual military tactic of 20th century insurgents.
Previous studies have largely focused on only one of these explanations, and often not done rivaling explanations justice when interpreting empirical evidence. This study is the first to systematically assess their relative strength through a theoretical and empirical analysis. In the theoretical part I discuss plausible lower-level arguments related to each explanation. By specifying the causal structure of these arguments I derive several hypotheses which help to distinguish between the explanations. These are tested using logistic regression analysis of quantitative data for 161 countries from 1945 to 1999.
My main conclusion is that the ecological explanation appears to be the strongest of the four explanations. Two of the ecological variables - the proportion of people living in urban areas and the extensiveness of telecommunication infrastructure - can empirically account for the entire per capita income effect on the risk of civil war. The two most prominent explanations in recent quantitative literature, state weakness and greed, find surprisingly little empirical support. Only one of the grievance hypotheses was to some extent supported by the evidence.