While the percentage of urban residents is higher in relatively developed countries, developing countries have an increasing number of very large urban settlements. In 1950, only three of the ten largest urban areas in the world were in developing countries, compared with six in the 1990s. It is difficult to rank the world's cities in order of population, because the definition of what constitutes an urban area different from one country to another and because up-to-date figures are rarely available, even in relatively developed countries. By utilizing different methods of delineating boundaries, London, Mexico City, New York, and Tokyo have all claimed in recent years to be the world's largest urban area. In 1991, the Population Crisis Committee applied a consistent definition to estimate the population of cities around the world and concluded that the Tokyo-Yokohama region in Japan was the world's largest urban area, followed by Mexico City and New York. That places in developing countries dominate the list of largest urban areas is a remarkable event, since urban growth has been historically associated with relatively developed economies. Urban settlements grew because newly create jobs in factories, shops, and offices attracted migrants from the countryside. But urban settlements in today's developing countries do not have rapidly expanding employment opportunities. People are migrating from rural to urban areas because of very poor economic conditions in rural settlements rather than realistic prospects for jobs in the cities. The rapid growth of urban areas in developing countries is also partly a reflection of an overall increase in population. The new residents in the rapidly growing urban areas of the developing world generally live in poor conditions, with as many as half the people residing in squatter settlements consisting of illegally erected shacks and Tents. These people lack urban services, such as running water, electricity, paved streets, transportation, schools, and shops. Immigrants to New York and London may have lived in squalid conditions, but an expanding economy at least provided them with jobs, even if rather menial and poorly paid. An adequate supply of jobs Is simply not available in Mexico City, Sao Paulo, and the other large urban settlements of today's developing countries. According to the passage, which is the real reason for people to move from rural to urban areas?