Journal article: This paper offers an explanation for the convergence of fertility rates across French départements in the second half of the nineteenth century that emphasises the diffusion of information through internal migration. It tests how migration affected fertility by building a decennial bilateral migration matrix between French départements for 1861‐1911. The identification strategy uses exogenous variation in transportation costs resulting from the construction of railways. The results suggest that the convergence towards low birth rates can be explained by the diffusion of cultural and economic information pertaining to low‐fertility behaviour by migrants, especially by migrants to and from Paris.