Fixed Effects and Post-Treatment Bias in Legacy Studies

A growing literature examines how historical institutions influence contemporary political attitudes and behavior. Recent work has argued that these studies need to properly account for spatial heterogeneity by incorporating regional fixed effects. Here, we discuss the theoretical and empirical obstacles that have to be addressed to properly incorporate fixed effects in legacy studies. We illustrate our arguments using Pepinsky et al.'s (2020) reassessment of a recent study on the long-term effects of concentration camps in Germany (Homola et al. 2020). We show that Pepinsky et al. incorporate fixed effects incorrectly and, as a result, their analysis suffers from post-treatment bias. We further demonstrate that results from the original article remain substantively the same when we incorporate regional fixed effects correctly. Finally, simulations reveal that camp proximity consistently outperforms spatially correlated noise in this specific study.