Popular tales of ¡°Mrs. Watanabe¡± ¡ª the canny Japanese housewife who dabbles in currency trading in between school runs and shopping ¡ª barely begin to tell the story of the nation¡¯s retail traders in the foreign exchange market.

With almost 800,000 active forex accounts, Japan boasts the world¡¯s most powerful force of retail traders. It has doubled in size in little more than a decade and spurred some of the most dramatic price moves of recent times, including the January ¡°flash crash¡± that hammered the dollar and sent the yen soaring.

Contrary to the widespread notion of ¡°Mrs. Watanabe,¡± most of the traders are middle-age men, who¡¯ve been driven into the market by years of ultra-low interest rates. They toil in offices by day and trudge home to moonlight in foreign exchange, hoping to build a family nest egg in a country where banks pay savers next to nothing.