Text 2 　　Buy land, advised Mark Twain; they’re not making it any more. In fact, land is not really scarce: the entire population of America could fit into Texas with more than an acre for each household to enjoy. What drives prices skyward is a collision between rampant demand and limited supply in the great metropolises like London, Mumbai and New York. In the past ten years real prices in Hong Kong have risen by 150%. Residential property in Mayfair, in central London, can go for as much as £55,000 ($82,000) per square meter. A square mile of Manhattan residential property costs $16.5 billion. 　　Even in these great cities the scarcity is artificial. Regulatory limits on the height and density of buildings constrain supply and inflate prices. A recent analysis by academics at the London School of Economics estimates that land-use regulations in the West End of London inflate the price of office space by about 800%; in Milan and Paris the rules push up prices by around 300%. Most of the enormous value captured by landowners exists because it is almost impossible to build new offices to compete those profits away. 　　The costs of this misfiring property market are huge, mainly because of their effects on individuals. High housing prices force workers towards cheaper but less productive places. According to one study, employment in the Bay Area around San Francisco would be about five times larger than it is but for tight limits on construction. Add up these costs in lost earnings and unrealized human potential, and the figures become dizzying. Lifting all the barriers to urban growth in America could raise the country’s GDP by between 6.5% and 13.5%, or by about $1 trillion-2 trillion. It is difficult to think of many other policies that would yield anything like that. 　　Two long-run trends have led to this fractured market. One is the revival of the city as the central cog in the global economic machine. In the 20th century, tumbling transport costs weakened the gravitational pull of the city; in the 21st, the digital revolution has restored it. Knowledge-intensive industries such as technology and finance thrive on the clustering of workers who share ideas and expertise. The economies and populations of metropolises like London, New York and San Francisco have rebounded as a result. 　　What those cities have not regained is their historical ability to stretch in order to accommodate all those who want to come. There is a good reason for that: unconstrained urban growth in the late 19th century fostered crime and disease. Hence the second trend, the proliferation of green belts and rules on zoning. Over the course of the past century land-use rules have piled up so plentifully that getting planning permission is harder than hailing a cab on a wet afternoon. London has strict rules preventing new structures blocking certain views of St Paul’s Cathedral. Google’s plans to build housing on its Mountain View campus in Silicon Valley are being resisted on the ground that residents might keep pets, which could harm the local owl population. Nimbyish residents of low-density districts can exploit planning rules on everything from light levels to parking spaces to block plans for construction.