AMERICA depends less on international trade than many other countries. Its imports and exports of goods add up to less than 25% of GDP; China’s are nearly 50%, Ireland’s, nearly 90%. It is not that Americans are insular; only that their vibrant domestic market is so big. The Brookings Institution, a think-tank, has assembled data on how America’s metropolitan areas trade (see chart). It finds that America moved $20 trillion in merchandise in 2010, or more than 100% of GDP, if trade between cities is counted. The top 20 metros account for almost a third of the total. St Louis and Detroit are busier traders than the Netherlands, a famously seafaring nation. And America’s metropolitan trade is not just in basic goods such as oil and wheat; nearly 30% of it is in “advanced industry” goods.