The impact of the two main religions on demographic behaviour until the middle of the 20th century is striking and, depending on religious affiliation, remarkable differences can be observed with regard to celibacy, fertility and size of family. In Catholic regions all three were much higher at the end of the 18th century already. As from the middle of the 19th century migration increased considerably to Protestant towns and industrialised regions from within the country and from abroad and this included a large number of Catholics. Migration affected both the migrant and the indigenous populations. On the one hand those immigrants coming from regions practising little or no birth control adopted the family limitation characteristic of the local population after a period of adjustment and, on the other hand, immigrants practicing family limitation influenced the conservative agrarian Catholic population that received them. Nevertheless, important differences existed within each main religion and there was also some notable atypical demographic behaviour according to affiliation within Protestantism, as seen in Pietism, and to a certain extent, too, within the Catholic population, depending on their political views. The erosion both of the “State” religion and of the influence of the Catholic Church as from the end of the 19th century contributed above all in urban and industrialised contexts to an increase in the proportion of mixed marriages and to a process of secularisation which resulted in a significant reduction in fertility.