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World Population Growth and Distribution The United Nations, an accepted authority on population levels and trends, estimate

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问题                        World Population Growth and Distribution
    The United Nations, 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.
A. Past and Present Growth
    Estimates of world population before 1900 are based on fragmentary (零散的) 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 (摆动) 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 labor. 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 (疫苗), 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 nationsa--rate that doubles population size in 23 years.
B. 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.
C. 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 centers. Dealing with these conditions, especially in very large cities, presents massive difficulties for the governments of less-developed countries.
D. Population Projections
    Most of the potential parents of the next two decades have already been born. Population projections over this interval can, therefore, be made with reasonable confidence, barring catastrophic changes. Beyond two decades, however, uncertainties about demographic magnitudes and other characteristics of human societies build up rapidly, making any projections somewhat speculative.
    Projections issued in 2000 show the world population increasing from 6.1 billion in 2000 to 7.9 billion in 2025 and 9.3 billion in 2050. "High" and "low" projections for 2025 are 8.4 billion and 7.5 billion respectively. The average world birth rate is projected to decline from the 1990 level of 26 per 1,000 to 22 per 1,000 at the end of the century and to 17.6 per 1,000 in 2025. Because of the expanding share of the population at high-mortality ages, the average world death rate is expected to decline only slightly; from 9 per 1,000 in 1990 to 8.4 in 2025. Average world life expectancy, however, is projected to rise from 65 years in 1990 to 71.3 years in 2025.
    Wide variations in population growth will undoubtedly persist. In the developed world, population growth will continue to be very low and in some nations will even decline. Western Europe as a whole is projected to have a declining population after 2000. U.S. Census Bureau projections, assuming middle fertility and mortality levels, show U.S. population increasing from 250 million in 1990 to 349 million in 2025 and 420 million in 2050. Thereafter, growth would be virtually zero.
POPULATION POLICIES
    Government population policies seek to contribute to national development and welfare goals through measures that, directly or indirectly, aim to influence demographic processes--in particular, fertility and migration. Examples include statutory minimum ages for marriage, programs to promote the use of contraception, and controls on immigration. (When such policies are adopted for other than demographic reasons, they can be termed implicit policies.)
A. Population Policy in the United States
    The early immigrants to North America found a vast continent with a relatively small indigenous population. Overcrowding was incomprehensible because of the expanse of land to the west.
    In the mid-20th century, as the rest of the world awakened to the potential crisis brought on by unchecked population growth, the U.S. government examined the possible impact of overpopulation in the nation. The President’s Commission on Population Growth and the American Future began a two-year study in 1970. Submitted to President Richard M. Nixon in 1972, it welcomed the prospect of zero population growth in the U.S., but did not propose that the government take strong measures to attain it. The commission did, however, advocate education on family planning and widely available access to contraception and abortion services. Primarily because of this, the president rejected the commission’s recommendations.
B. Population Policies in Developed Nations
    European countries did not address the issue of a national population policy until the 20th century. Subsidies were granted to expanding families by such disparate nations as the United Kingdom, Sweden, and the USSR. The Italian Fascists in the 1920s and the National Socialists (Nazis) in Germany during the 1930s made population growth an essential part of their doctrines.
    Japan, with an economy comparable to those of the European nations, was the first developed country in modem times to initiate a birth-control program. In 1948 the Japanese government formally instituted a policy using both contraception and abortion to limit family size.
C. Population Policies in the Third World
    In 1952 India took the lead among developing nations in adopting an official policy to slow its population growth. India’s stated purpose was to facilitate social and economic development by reducing the burden of a young and rapidly growing population. Surveys to ascertain contraceptive knowledge, attitude, and practice showed a high proportion of couples wishing no more children. Few, however, practiced efficient contraception. Family-planning programs were seen as a way to satisfy a desire for contraception by a large segment of the population and also to confer health benefits from spacing and limiting births.
    Asia’s lowered growth rate can be attributed mainly to the stringent (严厉的) population policies of China. Although it has a huge population, China has successfully reduced both fertility and mortality. The government has recently been advocating one-child families to lower the nation’s growth rate.
Most migrants to the cities can be assumed to have bettered themselves in comparison to ______.
选项
答案their former standard of living
解析 参见小标题Urban Concentration下面的第3段：Most migrants to the cities can be assumed to have bettered themselves in comparison to their former standard of living．由此可知，流动到城市的大多数的人可以认为与他们以前的生活标准相比，他们能够过得更好。
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