This chapter examines the uses of process tracing for empirically testing ideational explanations and theories of political decision-making. Ideational mechanisms have characteristics that make them especially difficult to study, as compared to materially driven causal processes. Ideas are unusually difficult to measure and are often highly correlated with other plausible causes of political outcomes. Moreover, key mechanisms of ideational influence operate within a “black box” of unobservability from the perspective of the historical researcher. These challenges of ideational analysis motivate this chapter’s arguments in two respects. On one level, the chapter seeks to demonstrate that process tracing represents an especially powerful empirical approach for distinguishing between ideational and material effects. At the same time, the chapter reckons with the considerable challenges that the study of ideational causation presents, even for careful process tracing.

The chapter offers ideational analysts a set of process tracing strategies as well as guidance in identifying the conditions under which each strategy can be fruitfully applied. Broadly, it emphasizes three hallmarks of effective tracing of ideational processes. The first of these is “expansive empirical scope.” It is tempting for analysts testing ideational explanations to zero in on key moments of political decision, on the handful of elite actors who were “at the table,” and on the reasons that they provided for their choices. However, for reasons outlined below, a narrow focus on critical choice points will rarely be sufficient for distinguishing ideational from alternative explanations. To detect ideational effects, our analytic field of view must be expansive in terms of both temporal range and level of analysis. A well-specified theory of ideas will imply predictions not just about individual elites’ statements and behavior at key moments of choice, but also about continuity and change, sequences of events, flows of information, and movements of actors across institutional settings over time.