SOME 52% of the world's population currently live in urban areas; by 2025 this should increase to 58%. Nearly all this growth will take place in emerging-market economies, particularly Asia, as migrants from the countryside move in search of jobs. Today, these emerging-market city-dwellers account for more than 60% of the world's GDP growth. In the past 15 years Delhi's population has grown by 10m; it will add another 6m (a Miami's worth) in the next 15. This puts ever more strain on city infrastructure, increasing crowds on transport, worsening water scarcity and exacerbating electricity shortages. BCG, a consultancy, reckons that over the next two decades urban areas in emerging economies will account for about two-thirds of worldwide infrastructure spending. Most will go on building homes. China and India together will add 16 trillion square metres of housing over this period, to accommodate half a billion extra people.