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for the outcome) there are six possible types. (There are not eight types because the order of cases in the mixed dyads does not matter.) Most of the research on the democratic peace has focused on one type of case: dyads that are by some measures democratic but nonetheless go to war against one another (or the close cousins of these cases, near-democracies that go to war, and near-wars between democracies). The focus on these cases at the early stages of the research program is appropriate, but depending on the nature of the hypothesized causal mechanisms, it may also be important to make comparisons to other kinds of cases. For example, do conflict resolution mechanisms between democracies differ from those between other kinds of dyads, including democracy/nondemocracy dyads and nondemocracy/nondemocracy dyads?497

This example suggests a second criterion for reducing the property space and choosing the specific cases to study from among the types that remain. When an outcome is overdetermined by existing theories and it turns out as expected, it is less likely to be theoretically informative—although process-tracing might show that causal mechanisms did not operate exactly as expected. Such most-likely cases are usually useful only when a theory unexpectedly fails to explain them, although process-tracing of most-likely cases with variables at extreme values can allow the researcher to see in stark relief how the underlying causal mechanisms operate.498

A third criterion for reducing the property space is the identification of which types and cases are suited to the research objective. This is true whether this objective is to test existing theories, compare typologically similar cases, identify and study deviant cases, or conduct a plausibility probe. The research objective and the case study research design should be devised with a view toward a research program’s stage of development. For example, new and relatively untested research programs are more likely to be advanced by plausibility probes and inductive studies of deviant cases. More advanced research programs may offer few or no clear deviant cases, but may be amenable to theory-testing case studies and studies of typologically similar cases with slightly different outcomes that might yield new subtypes or more finely differentiated variables.

From Property Space to Research Design

There are four different research designs that can reconcile a scholar’s research objective, the alternative research designs available, and the actual historical cases available for study. These include comparing similar or differing cases in the same type; comparing most similar cases in adjacent types with differing outcomes; studying most-likely, least-likely, and crucial cases; and comparing least similar cases.

CASES IN THE SAME TYPE: CASES THAT VALIDATE THE TYPE VS. DEVIANT CASES

In the first research design, if two cases fit into the same type according to their independent variables, our working assumption is that they should have similar outcomes. This offers the most basic test of the validity of the specification of the type. If a preliminary classification of cases into their respective types indicates that cases in the same type have different outcomes, then the researcher can perform full studies of these cases to assess whether and why one of them deviates from the expected outcome. This can uncover errors in the preliminary measurement and classification of one or both cases, or it may point to additional variables that deserve attention.499 Even if a change in the measurement of one of the cases or the addition of a new variable resolves the anomaly in the type in question, it may create another anomaly in another type, which then requires investigation.

MOST SIMILAR CASES: ADJACENT TYPES WITH DIFFERING OUTCOMES

A second potential research design arises when the preliminary classification of the cases indicates that two cases differ in only one independent variable and also in the dependent variable. This allows a most-similar cases research