There are four main stages that have been identified in population growth. In Stage 1, both birthrates and death rates are high. Observed historical situations show that at this stage, medical technology is not developed enough to lengthen life. Personal hygiene also limits life expectancy. The overall rate of births and deaths fluctuate depending on the effects of outside circumstances such as disease and natural disasters.
In stage 2 of the population growth cycle, overall death rates begin to decline. Hygiene and the appearance of more advanced medical treatment reduce the detrimental effects of disease on the population. An upward trend in population size begins, and this stage also marks the beginning of what is known as demographic transition. This transition denotes a worldwide process that has occurred over the past century, in which population growth stabilizes in developed countries. As the cycle moves into stage 3, there is the tendency for people to move into urban areas, in which supporting a large family becomes too costly and birthrates decrease. There is a decrease in childhood mortality and families have fewer children, reducing the overall birthrate. The population continues to grow since the number of women of childbearing age is still high.
Birth and death rates remain low as stage 4 of demographic transition begins. With a near equilibrium established, the population grows slowly if at all. At this stage, the only means for significant population growth in an area is a liberal immigration policy. Most of the developed world, such as the United States and Europe, are in stage 4. Other regions are experiencing other stages, such as sub-Saharan Africa, which is experiencing rapid growth and a high death rate, and is in stage 1 or 2. Latin America is mostly in stage 3, with dropping birthrates but having a relatively young population.
The process of population growth can only be measured based on past analysis. The growth rate for developed countries has declined to less than 0.3 percent and is 6 times that rate in other parts of the world. Worldwide, the population growth rate is accelerating. The population doubled in the 40 years after 1950, to 5 billion, and is expected to top 6 billion by the end of the 20th century. It is estimated that the world population could reach 12 billion by the late 21st century. Scientists continuously debate how many people the Earth can support and theorize on a concept know as carrying capacity, the limit of how many people the planet can sustain.