IN THE second quarter of the year the global economy grew at its slowest rate since the end of 2009, according to The Economist's measurement, which is based on an assessment of 52 countries. This is mostly due to slower growth in America and China, and to the accelerating shrinkage of the euro-zone economy. In 1990 the US accounted for a quarter of the world’s GDP and China just 4%. Today America’s share is less than one-fifth, while China’s has expanded to 15%. Yet in the latest quarter America made its biggest contribution to world GDP growth since 2005 (excluding periods of global recession). Such changes in economic heft are reflected in the weights used to calculate global growth.