**Acknowledgements**

This work benefited from a grant from the French National Research Agency under the "Investments for the Future" program bearing the reference "ANR-10-LABX-1401." The author thanks in particular FERDI (the French Foundation for Research on International Development) for its support and interest in the subject, as well as Mr. Charles Becker (historian, CNRS, France) and Dr. Monica Das Gupta (demographer, The World Bank, USA), for their careful reading of the manuscript, their comments and suggestions

