Countries in Northwestern Europe, including Belgium, report cohort fertility levels of close to two children per woman; whereas Central European countries, such as Germany, have levels of around 1.6 children. In seeking to explain these differences, some scholars have stressed the role of the social policy context, while others have pointed to variation in fertility-related social norms. But because these influences are interdependent, it is difficult to isolate their effects on fertility trends. This study attempts to disentangle these two factors by drawing on a quasi-natural experiment. After World War I Germany was compelled to cede the Eupen–Malmedy territory to Belgium. The population of this region has retained its German linguistic identity, but has been subject to Belgian social policies. We examine whether the fertility trends in this German-speaking region of Belgium follow the Belgian or the German pattern. Our findings indicate that they generally resemble the Belgian pattern. This suggests that institutional factors are important for understanding the current fertility differences in Western Europe.