A startling and profoundly important fact about the US economy has received surprisingly little attention. The educational quality of the country's workers is starting to decline - not just relatively (because other countries are catching up and moving ahead) but also, for the first time, in absolute terms. Over the coming years, baby-boomers departing from the labour force will have better educational qualifications than the younger workers replacing them. If the ultimate source of an economy's ability to grow and prosper is its human capital, the US is in trouble.

For decades the educational quality of the US labour force surged. In 1940, less than 5 per cent of the population aged 25-64 had at least a four-year college education. By 2000, the proportion had increased to nearly 30 per cent. Successive generations of workers improved on the educational attainments of their predecessors. Retiring workers were replaced by better-educated youngsters. This remorseless accumulation of human capital helped fuel the country's postwar growth. According to at least one authoritative study, it was the principal driver.