In 2008, the world's urban population passed its rural population for the first time in history. And, according to a United Nations study , there is no turning back: Between 2007 and 2050, the world population is expected to increase by 2.5 billion, passing from 6.7 billion to 9.2 billion (United Nations, 2008). At the same time, the population living in urban areas is projected to gain 3.1 billion, passing from 3.3 billion in 2007 to 6.4 billion 2050 (see figure at right). Thus, the urban areas of the world are expected to absorb all the population growth expected over the next four decades while at the same time drawing in some of the rural population. As a result, the world rural population is projected to start decreasing in about a decade and 0.6 billion fewer rural inhabitants are expected in 2050 than today. Furthermore, most of the population growth expected in urban areas will be concentrated in the cities and towns of the less developed regions. Asia, in particular, is projected to see its urban population increase by 1.8 billion, Africa by 0.9 billion, and Latin America and the Caribbean by 0.2 billion. Population growth is therefore becoming largely an urban phenomenon concentrated in the developing world. Just a quarter century ago, the world's urban population was just 40.9 percent of the total global population. And these last 25 years were the height of globalization--so what happened? Or, as Harvard economist Edward Glaeser writes on the New York Times Economix blog puts it, "If the world is so flat, then why are cities growing so quickly, especially in the third world?" Glaeser--answering his own question--points to the nature of the global economy as one that is built on technology. While new technology makes it possible in the information economy to work from anywhere, humans thrive on interaction, and we place great value on proximity: Globalization and technological change have increased the returns to being smart; human beings are a social species that get smart by hanging around smart people. A programmer could work in the foothills of the Himalayas, but that programmer wouldn’t learn much. If she came to Bangalore, then she would figure out what skills were more valuable, and what companies were growing, and which venture capitalists were open to new ideas in her field. The information flows that come from proximity might also help to build the relationships that would enable her to create her own start-up. A remarkable number of information-technology start-ups in India were formed by partners who connected in Bangalore. Read Glaeser's post here . (Hat tip to the either early-rising or late-to-bed Mark Thoma for highlighting Glaeser's piece while we were asleep).