The exit of the United Kingdom from the European Union (“Brexit”) was a seismic, and largely unexpected, political and economic event. In a recent contribution to this journal, Mavropoulos (2024) used difference-in-difference (DID) estimation to examine the impact of Brexit on fertility dynamics among European countries and concludes that there are large and robust Brexit effects. DID estimation is a tool for extracting quasi-causal estimates from observational data. As an analytic strategy, it has a number of advantages over traditional regression or covariate adjustment approaches. Using Mavropoulos as a reference point, this article re-visits the Brexit and fertility question using data from the World Bank database to elaborate on the complexities of DID estimation and the critical role of “researcher degrees of freedom”—the decisions that DID analysts must make to satisfy the underlying assumptions of the approach. Empirically, we show the important role of careful selection of control cases and ultimately conclude that there is little support for the ”Brexit hypothesis” with theoretical and methodological issues discussed.
