"Demography is the discipline whose primary concern is measuring the size and composition of populations, and the changes in size and composition of populations over time. It is sometimes used to encompass, in addition, the causes and consequences of demographic changes on other important social processes such as economic development. Efforts by states and governments to collect information on population size, composition, and dynamics are millennia old, and were motivated by the need to tax populations, form armies, detect epidemics, form viable political and socio-economic units, and provide adequate social services such as policing, health services, and schools. Demography is no less relevant today. Many of the key challenges of development in Africa and the rest of the world are locked in complex relationships of mutual cause and effect with demographic processes."