On Monday the world population reached seven billion, according to the United Nations. By 2050, there are expected to be 9.2 billion people on the planet and around 75 per cent of them will be living in cities - up from the current 53 per cent. Metropolitan areas in many of the world's developing nations are experiencing exponential growth. It's estimated that up to 60 people move to Nigeria's capital Lagos every hour. In some cities, planners respond by tearing down unsightly slums, dispersing the newcomers. But academics think that's the wrong approach.
