Mill’s Method of Agreement and Method of Difference as Methods of Analysis in International Relations

John Stuart Mill’s Method of Agreement and Method of Difference have been well known both in international relations (IR) scholarship and methods discussions. Despite numerous methodological innovations since the publication of Mill’s A System of Logic in 1843, the persuasive logic and intuitive appeal of the Method of Agreement and the Method of Difference have allowed them to remain a staple in IR scholarship. The utility of Mill’s methods, however, has not gone unquestioned. Claims persist that the two methods are not suitable for social science research because their requirements are too demanding. Sustained criticism of Mill’s methods requires a sober look at their promise and potential to facilitate causal inference. Cautionary notes necessitate awareness of the logic of Mill’s comparative methods, their underlying assumptions, and remedies for potential pitfalls and weaknesses. Three underlying assumptions are associated with Mill’s methods—determinism, the inclusion of all causally relevant causes, and the independence of cases. Three core criticisms have been leveled at Mill’s methods: the inability to deal with equifinality, the narrow focus on single causes, and the purported incompatibility with observational research. Causal inference in observational settings can be strengthened by dealing with interaction effects, by specifying causal connections as correlational or set-relational, by supplementing the use of Mill’s methods with within-case procedures like process tracing, and by using qualitative comparative analysis (QCA) as a similar yet more complete comparative method.