President Obama’s recent trip to China reflects a symbiotic relationship at the heart of the global economy: China uses American spending power to enlarge its private sector, while America uses Chinese lending power to expand its public sector. Yet this arrangement may unravel in a dangerous way, and if it does, the most likely culprit will be Chinese economic overcapacity. Several hundred million Chinese farmers have moved from the countryside to the cities over the last 30 years, in one of the largest, most rapid migrations in history. To help make this work, the Chinese government has subsidized its exporters by pegging the renminbi at an unnaturally low rate to the dollar. This has supported relatively high-paying export jobs; additional subsidies have included direct credit allocation and preferential treatment for coastal enterprises. These aren’t the recommended policies you would find in a basic economics text, but it’s hard to argue with success. Most important, it has given many more Chinese a stake in the future of their society. Those same subsidies, however, have spurred excess capacity and created a dangerous political, dynamic in which these investments have to be propped up at all cost. China has been building factories and production capacity in virtually every sector of its economy, but it’s not clear that the latest round of investments will be profitable anytime soon. Automobiles, steel, semiconductors, cement, aluminum and real estate all show signs of too much capacity. In Shanghai, the central business district appears to have high vacancy rates, yet building continues. Chinese planners now talk of the need to restrict investment in sectors that are overflowing with unsold products. The global market is no longer strong, and domestic demand was never enough in the first place. Regional officials have an incentive to prop up local enterprises and production statistics, even if that means supporting projects or ac- counting practices that are not sustainable.