Measures of population
The numbers of births, deaths, immigrants, and emigrants over a specified time interval determine the change in population size. For comparative purposes, these components of change are expressed as proportions of the total population, to yield the birth rate, death rate, migration rates, and the population growth rate. (Birth and death rates typically are stated as numbers per 1,000 population per year.) These rates are affected by the age-composition of the population; for example, a very healthy population, which, as a result, has a relatively large proportion of old people, might have a death rate similar to that of a poor population made up of predominantly younger members. Demographers, therefore, often use measures that are free of this age-distribution influence. Two such widely used measures are the total fertility rate (TFR) and the life expectancy at birth.
The total fertility rate is the number of children a woman would have during her reproductive life if she experienced the prevailing rates of fertility at each age. High-fertility countries may have birth rates of 40 or even 50 per 1,000 population (per year); corresponding levels of the TFR would be 5 to 7 children per woman. Low-fertility countries have birth rates of 15 to 20 per 1,000 and TFRs of about 2. “Replacement level” fertility (the level at which each person on average has a single successor in the next generation) corresponds to a TFR of about 2.1 under low-mortality conditions.
The life expectancy at birth is the average length of life that would be observed in a population in which the currently prevailing mortality risks at each age continued indefinitely. Preindustrial populations were characterized by large fluctuations in mortality; long-run averages, however, would probably have shown death rates of 30 to 40 per 1,000 and life expectancies of 25 to 35 years. Under modern health conditions, death rates below 10 per 1,000 and life expectancies above 70 years are common.
Another important mortality measure is the infant mortality rate. This is the probability of death in the first year of life, usually stated as a number per 1,000 births. Many less-developed countries have infant mortality rates above 100 per 1,000—that is, more than 10 percent of the children die in their first year. In countries with effective health and educational systems, infant mortality rates are about 15 per 1,000, or even lower.
World population and distribution
The United Nations (UN), an accepted authority on population levels and trends, estimates that the world population reached 6 billion in 1999, and is increasing annually by more than 77 million persons. The rate of increase, 1.3 percent per year, has fallen below the peak rate of 2 percent per year attained by 1970. By the late 2040s, the UN estimates, the growth rate will have fallen to about 0.64 percent annually, at which time more than 50 countries will experience negative growth.
Past and present growth
Estimates of world population before 1900 are based on fragmentary data, but scholars agree that, for most of human existence, long-run average population growth approached approximately 0.002 percent per year, or 20 per million inhabitants. According to UN estimates, the population of the world was about 300 million in the year AD 1, and it took more than 1,500 years to reach the 500 million mark. Growth was not steady but was marked by oscillations dictated by climate, food supply, disease, and war.
Starting in the 17th century, great advances in scientific knowledge, agriculture, industry, medicine, and social organization made possible rapid acceleration in population growth. Machines gradually replaced human and animal labour. People slowly acquired the knowledge and means to control disease. By 1900 the world population had reached 1.65 billion, and by 1960 it stood at 3.04 billion.
Beginning about 1950, a new phase of population growth was ushered in when famine and disease could be controlled even in areas that had not yet attained a high degree of literacy or a technologically developed industrial society. This happened as a result of the modest cost of importing the vaccines, antibiotics, insecticides, and high-yielding varieties of seeds produced since the 1950s. With improvements in water supplies, sewage-disposal facilities, and transportation networks, agricultural yields increased, and deaths from infectious and parasitic diseases greatly declined. Life expectancy at birth in most developing countries increased from about 35-40 years in 1950 to 66 years by 2000. The rapid decline in deaths among people who maintained generally high fertility rates led to annual population growth that exceeded 3.1 percent in many developing nations—a rate that doubles population size in 23 years.
Regional distribution
As of 2000, 1.2 billion people lived in the developed nations of the world, and 4.9 billion people lived in the less-developed countries. By region, over half the world's population was in East and South Asia; China, with 1.3 billion inhabitants, and India, with some 1 billion, were the dominant contributors. Europe and the countries of the former USSR contained 14 percent, North and South America made up 14 percent, Africa had 13 percent, and the Pacific Islands had about 1 percent of world population.
Differences in regional growth rates are altering these percentages over time. Africa's share of the world population is expected to more than double by the year 2025. The population of South Asia and Latin America is expected to remain nearly constant; in other regions, including East Asia, the population is expected to decline appreciably. The share of the present developed nations in world population—20 percent in 2000—is expected to fall to 15 percent by 2025. Nine out of every ten persons who are now being added to the world's population are living in the less-developed countries.
Urban Concentration
As a country develops from primarily an agricultural to an industrial economy, large-scale migration of rural residents to towns and cities takes place. During this process, the growth rate of urban areas is typically double the pace of overall population increase. Some 29 percent of the world population was living in urban areas in 1950; this figure was 43 percent in 1990, and is projected to rise to 50 percent by the year 2005.
Urbanization eventually leads to a severe decline in the number of people living in the countryside, with negative population growth rates in rural areas. Rapid growth of overall population has deferred this event in most less-developed countries, but it is projected to occur in the early decades of the 21st century.
Most migrants to the cities can be assumed to have bettered themselves in comparison to their former standard of living, despite the serious problems of overcrowding, substandard housing, and inadequate municipal services that characterize life for many arrivals to urban centres. Dealing with these conditions
, especially in very large cities, presents massive difficulties for the governments of less-developed countries.
World population today: 6,308,573,202