This MOOC aims to answer to the following question: “How to explain the global decrease in fertility from a socio-demographic perspective?” The objective is to explain the complexity of a fundamental human behavior, fecundity, by showing that a multi-disciplinary approach is needed and not only a focus on the data itself. It shows that demographic evolutions make sense only in the view of economical, social, cultural and political factors, widely intertwined. This MOOC is associating theory and concrete case study, often from personal field research in Africa, Asia and Latin America.