As mortality rates decline, people live longer and contribute to higher population levels. During the 20th century, global average lifespan increased from approximately 40 years to approximately 67 years. Life expectancy varies widely throughout the world, being higher in the Americas, Europe, Australia, China and Japan, and lower in Africa, India, Russia and central Asia. But even the poorest countries of the world have seen significant improvements in mortality rates.
The world's fastest-growing human populations are in poor countries. Total fertility rate, or TFR, measures the total number of children born per woman in each country. The countries with the ten highest fertility rates are in Africa. The top 20 countries have fertility rates above five. Moreover, each of these countries suffers from low per capita income levels and high rates of poverty. Looking at the data by country, the link between poverty and rapid population growth is indisputable.
Economically insecure countries tend to have high child mortality rates. Moreover, the poorest countries rely heavily on subsistence agriculture. In these cultures, children become economically productive at an early age. Given the threats from disease and malnutrition, families have an incentive toward larger family sizes to ensure that some children reach adulthood. Larger families increase economic security, especially for the elderly who rely almost exclusively on offspring for economic support.
Population has stabilized across much of the developed world, with many countries experiencing shrinking native populations. Some developed countries, such as the United States, continue to see population increase mainly as a result of immigration. The total fertility rate in the United States is 2.1, about the level required for maintaining a stable population. According to the U.S. Census Bureau, U.S. population is forecast to rise to 420 million in 2050, almost exclusively due to immigration.