1. Vienna Institute of Demography (Austrian Academy of Sciences), A-1040 Vienna, and World Population Program, International Institute for Applied Systems Analysis (IIASA), Schlossplatz 1, A-2361 Laxenburg, Austria.
2. University of Innsbruck, A-6020 Innsbruck, and World Population Program, IIASA
3. Stony Brook University, Stony Brook, NY 11794, USA, and World Population Program, IIASA