1. MRC/WITS Developmental Pathways for Health Research Unit, Department of Paediatrics, School of Clinical Medicine, Faculty of Health Sciences, University of Witwatersrand, Johannesburg, South Africa;
2. Human and Social Development Research Programme, Human Sciences Research Council, Pretoria, South Africa;
3. MRC/Wits Rural Public Health and Health Transitions Research Unit, School of Public Health, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg Parktown, South Africa;
4. INDEPTH Network, Accra, Ghana;
5. Umeå Centre for Global Health Research, Umeå University, Umeå, Sweden;
6. School of Demography, The Australian National University, Acton, Australia;
7. Department of Paediatrics, University of Cambridge, Cambridge, UK