In historical demography, the impact of the religious factor on fertility is mentioned from time to time, but for Germany evidence is contradictory and often not significant. Until now, studies have led to the conclusion that regional, local and social factors provide sufficient explanations for the differences observed. This initial hypothesis is tested here for three Upper Rhine villages with a mixed population of Catholics and Protestants, for which micro-data from the so-called Ortssippenbücher (village genealogies) exist. Thanks to it, the geographical factor can be excluded as a source of diversity, and the nature of the social structure in the villages helps to distinguish between religious and social factors. The results oblige us to rethink the connection between social factors, the structure of communication, and the formation of confessional identity. Never before has such an early and consequent limitation of family size been observed in Germany as is the case here for Lutherans in the northern Ortenau. On the other hand, the number of children of Catholics was extremely high. This is true for the two main social categories. Denomination was obviously so strong a factor of identity that for centuries completely different modes of reproduction could coexist in the same village. Such a contrast was not just a local phenomenon, but probably of fundamental importance for the entire Upper Rhine valley. Finally, the results of our study open the perspective of an enlargement of Max Weber’s theory on Protestantism to the domain of demographics.