1. a Department of Sociology and Anthropology, University of Uyo, Uyo, Nigeria, and Demography and Population Studies Programme , University of the Witwatersrand , Johannesburg , South Africa
2. b Demography and Population Studies Programme , University of the Witwatersrand , Johannesburg , South Africa
3. c Department of Geography, School of Natural Sciences, University of Zambia, Lusaka, Zambia, and Demography and Population Studies Programme , University of the Witwatersrand , Johannesburg , South Africa
4. d Demography and Social Statistics Department, Obafemi Awolowo University, Ile-Ife, and Demography and Population Studies Programme , University of the Witwatersrand , Johannesburg , South Africa