By 2030 China's economy could loom as large as America's in the 1970s

A NEW book, discussed in this week's Economics focus, by Arvind Subramanian of the Peterson Institute for International Economics argues that China's economic might will overshadow America's sooner than people think. Mr Subramanian combines each country's share of world GDP, trade and foreign investment into an index of economic “dominance”. By 2030 China's share of global economic power will match America's in the 1970s and Britain's a century before. Three forces will dictate China's rise, Mr Subramanian argues: demography, convergence and “gravity”. Since China has over four times America's population, it only has to produce a quarter of America's output per head to exceed America's total output. Indeed, Mr Subramanian thinks China is already the world's biggest economy, when due account is taken of the low prices charged for many local Chinese goods and services outside its cities. China will be equally dominant in trade, accounting for twice America's share of imports and exports. That projection relies on the “gravity” model of trade, which assumes that commerce between countries depends on their economic weight and the distance between them.