Several countries in Northern and Western Europe report cohort fertility rates of close to two children per woman, including Belgium, France, and Denmark. By contrast, most Central and Southern European countries have cohort fertility levels of only around 1.5-1.6 children.Germany is part of this second group. In order to explain these country differences in fertility levels, some scholars have stressed the role of the social policy context, while others have pointed to differences in social fertility norms. However, due to the interdependence of these two factors, it is cumbersome to isolate their impact on fertility trends. In our study we attempt to disentangle these influences by drawing on a quasi-natural experiment. In the aftermath of World War I, Germany was forced to cede the territory of Eupen-Malmedy to Belgium. The population in this area retained its German linguistic identity, but has been subject to Belgian social policies since the early 1920s. Our main research question is whether the fertility trends in this German-speaking region of Belgium follow the Belgian or the German pattern more closely. To answer this question, we use (micro)-census data to compare the fertility behavior in the German-speaking region in Belgium with data for western Germany and the Belgian Flemish-and French-speaking regions, controlling for individual-level characteristics. Our findings indicate that the overall fertility outcomes of the German-speaking region in Belgium resemble the Belgian pattern more than the German one. This provides support for the view that institutional factors play an important role for understanding the current fertility differences in Western Europe.