1. Development Research Group, The World Bank, 1818 H. Street NW, Washington, DC 20433, USA
2. Health Nutrition and Population, The World Bank, 1818 H. Street NW, Washington, DC 20433, USA
3. DIAL-Paris Dauphine University, 4 Rue d'Enghien, 75010 Paris, France
4. Population Studies and Training Center, Brown University, 68 Waterman St, Providence, RI 02912, USA