Causal Effects, Migration, and Legacy Studies

Political scientists have long been interested in the persistent effects of history on contemporary behavior and attitudes. To estimate legacy effects, studies often compare people living in places that were historically exposed to some event and those that were not. Using principal stratification, we provide a formal framework to analyze how migration limits our ability to learn about the persistent effects of history from observed differences between historically exposed and unexposed places. We state the necessary assumptions about movement behavior to causally identify legacy effects. We highlight that these assumptions are strong; therefore, we recommend that legacy studies circumvent bias by collecting data on people's place of residence at the exposure time. Reexamining a study on the persistent effects of U.S. civil rights protests, we show that observed attitudinal differences between residents and nonresidents of historic protest sites are more likely due to migration rather than attitudinal change.