AMERICANS, and friends of America, often reassure themselves about its relative decline in the following way. Even if the roads, airports and schools continue to slide, it will retain its lead in the most sophisticated fields for decades. They include defence, elite universities, and, in the business world, technology. Uncle Sam may have ceded the top spot to China in exports in 2007, and manufacturing in 2011, and be on track to lose its lead in absolute GDP by about 2030.