Rapid urban growth is caused by three factors: migration to cities from rural areas; natural population increase among city residents and reclassification of previously rural areas as urban, as they became built up. All of these factors are more prominent in developing countries than developed countries, which is why changes in the largest cities locations, from the developed world to the developing world have occurred.
In developing countries rural to urban migration occurs at a very high rate because of deteriorating conditions in rural areas. Rapid population growth among low-income groups in rural areas has put a lot of pressure on resources such as agricultural land, forests, and water. These pressures have been made worse by large-scale export-oriented agriculture, loss of land possession among poor farmers, declining soil fertility and national policies that encourage agricultural increase and merging of farms. The shrinking size of family farms is common, as plots are divided into smaller and smaller pieces for each new and larger generation of male heirs. This means that many plots are no longer large enough to feed average families. At the same time, more and more agricultural land is being taken out of production because of soil degradation, a result mainly of wind and water erosion, loss of soil nutrients, and poor irrigation systems that cause salinisation and water logging. Farm living is dependent on unpredictable environmental conditions, and in times of drought, flood or plague, survival becomes extremely difficult. In many developing countries urban areas are magnets that attract people from small towns and the rural areas, because cities offer more hope of jobs, education, health care, and better living standards. Big cities in particular are economic, social, and cultural centres. People move to urban areas looking for better-paid jobs because urban areas have larger amounts and more variety of paid work. They are also hoping for a better chance of putting their children through school, a higher quality of life, access to services and more reliable food sources.
In developed countries rural conditions are much better and rural to urban migration is becoming increasing rare, and in some case is being reversed.
Natural increase accounts for about half of urban population growth. Natural increase is higher in urban areas in developing countries, due to improvements in medical care such as hospitals and medicines, improvements in sanitation and water supply, a rise in quality and quantity of food produced. Transport and communications improve the movements of food and medical supplies. Mortality decreases because of these reasons. Birth rates remain high in developing countries due to little access to birth control and for social reasons such as religious beliefs (e.g. Roman Catholics and Hindus) encouraging large families and children still being regarded as a sign of virility. Also in many developing countries children are still needed to work to earn money for the family. This natural increase doesn’t occur in developed cultures for several reasons; increased access to contraception, the desire for material possessions takes over the desire for large families as wealth increases and equality for women means that they are able to follow a career path rather than feeling obliged to have a family.
Political changes have also had profound influences on urban growth in the last 50 years. The break up of colonial empires helped to support rapid urban growth in most of Africa and Asia between the 1950s and the 1980s. One of the main reasons why many cities in Africa grew so rapidly during the 1960s and 1970s was that they began from such a small origin. This was largely because the colonial powers had restricted the rights of their national populations to live and work in urban centres. Another reason was the achievement of political independence and the development in urban centres of new government and educational institutions that created jobs and services. .