Demographic history has made a name for itself through works on the modern period, while the 19th and 20th Centuries have remained somewhat neglected by historians. The latter have to deal with multiple sources, and need to read them with a precise knowledge of how they were elaborated. This abundance of documents allows to give the first place to a differential study of the biological fate of several social categories, which reflects their way of life. If, beyond social history, the economic historian, as well as the one of international relations, must take into account the demographic factor, he must exclude all oversimplified determinism which would incline towards a populationist or a malthusian pattern. The demographic factor interact...