PIP: Due to a combination of rapid population growth and high levels of rural-urban migration, overcrowding will be common in many cities around the world in the 21st century. Currently at 5.3 billion, the global population is expected to increase to 6 billion by the year 2000, and to 9 billion by 2025. Experts predict that urban centers will bear the brunt of the population growth. Rural areas have seen declines in the standard of living, partly due to natural disasters, civil war, and economic policies favoring urban centers. In search of jobs, better access to education, and health services, rural populations will flock to cities. But the rapid growth of cities will inevitably lead to the creation of slums, which will hamper urban development. Urban demographers predict that by the end of the century, 1/2 of the world's population will be urban, and 1/5 of these people will be concentrated in "mega cities," populations of 4 million or more. International migration will play a significant role, as people cross borders in search of opportunity. But contrary to the traditional model of urban growth, much of it will take place in developing countries. According to a 1985 study, developed nations had an urbanization level of 71%, compared to 31% in developing countries. However, experts calculate that by 2025, these levels will practically even out, with an urbanization level of 74% for developing countries and 77% for developed countries. By 2025, 25 cities will have populations of over 9 million, including Mexico City (25.8), Sao Paulo (24.0), Tokyo (20.2), Calcutta (16.5), Greater Bombay (16.0), and New York (15.8).