FOR about 50 years after the second world war the combination of Japan's fast-growing labour force and the rising productivity of its famously industrious workers created a growth miracle. Within two generations the number of people of working age increased by 37m and Japan went from ruins to the world's second-largest economy. In the next 40 years that process will go into reverse. The working-age population will shrink so quickly that by 2050 it will be smaller than it was in 1950, and four out of ten Japanese will be over 65. Unless Japan's productivity rises faster than its workforce declines, which seems unlikely, its economy will shrink.