"To understand why some developing countries have progressively seen their fertility level declined and converged with the rest of the world during the twentieth century while others have not, this proposal offers an historical perspective based on France, which was the first European country where the fertility rate declined substantially.The secular decline in French fertility during the 19th century is a perennial puzzle as France was a relative laggard in urbanisation, education and social insurance. This research proffers an alternative explanation that takes into account the other French originality: most of the French emigrants were internal. Compared to the other Europeans, not many French moved to the high-fertility societies of America. This suggests a role for internal diffusion of fertility norms.Once migration and diaspora networks are formed, they reduce transaction and other types of information costs. They facilitate transactions between countries as well as the diffusion of technology and ideas. This suggests that the transfer of fertility norms prevailing in the host countries influence fertility choices in migrant-sending countries.The potential for cultural norm transmission is important as at the turn of the 20th century, France was not a fully integrated country from a cultural point of view. In some regions, a substantial share of the population still did not speak French and this language barrier reflected further cultural differences. Accordingly, the French fertility decline was not homogenous and the second half of the 19th century witnessed a strong convergence between the fertility of the various French regions.We hope that our research may explain to what extent migration played an important role in internal French fertility convergence. If similar phenomena are at play nowadays, they might explain the decline of fertility in developing countries."