Urbanization is a population shift from rural to urban areas, and the ways in which society adapts to the change. It predominantly results in the physical growth of urban areas, be it horizontal or vertical. The United Nations projected that half of the world's population would live in urban areas at the end of 2008. It is predicted that by 2050 about 64% of the developing world and 86% of the developed world will be urbanized.Urbanization is relevant to a range of disciplines, including geography, sociology, economics, urban planning, and public health. The phenomenon has been closely linked to modernization, industrialization, and the sociological process of rationalization. Urbanization can be seen as a specific condition at a set time (e.g. the proportion of total population or area in cities or towns) or as an increase in that condition over time. So urbanization can be quantified either in terms of, say, the level of urban development relative to the overall population, or as the rate at which the urban proportion of the population is increasing.Urbanization is not merely a modern phenomenon, but a rapid and historic transformation of human social roots on a global scale, whereby predominantly rural culture is being rapidly replaced by predominantly urban culture. The first major change in settlement patterns was the accumulation of hunter-gatherers into villages many thousand years ago. Village culture is characterized by common bloodlines, intimate relationships, and communal behavior whereas urban culture is characterized by distant bloodlines, unfamiliar relations, and competitive behavior. This unprecedented movement of people is forecast to continue and intensify in the next few decades, mushrooming cities to sizes unthinkable only a century ago. Today, in Asia the urban agglomerations of Dhaka, Karachi, Jakarta, Mumbai, Delhi, Manila, Seoul and Beijing are each already home to over 20 million people, while the Pearl River Delta, Shanghai-Suzhou and Tokyo are forecast to approach or exceed 40 million people each within the coming decade. Outside Asia, Mexico City, Sao Paulo, New York City, Lagos and Cairo are fast approaching being, or are already, home to over 20 million people.