Conditions, Necessary and Sufficient

Social science, as all science, is a continuous quest for an explanation and understanding of the world around us. This search is carried out through Benjamin Most and Harvey Starr's “research triad” of theory, logic, and research design, all of which are central to both hypotheses and results. The key element of explanation and understanding is causation. Social scientists are concerned with causation as applied both to individual events or cases and to classes or groups of events. The causal relationship may take many forms. Two of the most prominent, important, and commonly used forms of the causal relationship involve necessary and/or sufficient relationships. As emphasized in the work of Most and Starr among others, analyses must be concerned with the form of the relationship. David Hume's classic definition of cause involves the [Page 385]“constant conjunction” of an object followed by another, where all objects similar to the first are followed by objects similar to the second. Causation is seen as including three elements, the first being the existence of correlation between two factors or variables. That is, changes or attributes in one factor are associated with changes or attributes in another. Correlation is one possible form of any specific relationship. Correlation itself may take several forms, such as linear or curvilinear. The second element of cause involves the temporal dimension—the proposed causal factor must take place before the “effect” or the phenomenon to be explained. Cause must precede effect. The third element is the most rigorous and difficult—the elimination of other explanatory factors beyond those proposed (the central idea of “control”). Below, the various implications of these relationships and some current applications in political science are discussed.

Necessity and sufficiency are themselves different forms of the causal relationship. They represent different forms of constant conjunction. Scholars have argued that research designs (the nature of the theoretical logic and research hypotheses being employed) built around these different forms of causation will be affected by the nature and types of the cases selected, the controls employed for dealing with possible other (explanatory) factors, and the methods used for evaluating the theory and proposed research hypotheses. All three elements of the research triad—theory, logic, and research design—are thus affected by the form of the relationship, especially looking at necessary relationships in distinction from sufficient relationships or even simple correlational relationships. The differences between how necessary and sufficient conditions are treated are additionally important in regard to the research designs needed to investigate inference in the small-N studies that characterize much of comparative politics (and qualitative analyses more generally). The logics of necessary and sufficient conditions need to be investigated in order to cross the boundaries between political science subfields as well as the quantitative–qualitative divide.