New Delhi, May 6 -- More than two months on from the beginning of the third Gulf War, despite warnings of oil and gas shortages in the coming weeks, stock investors in the US have used bad news to buy on the dips. This is business as unusual: incredibly, the S&P 500 index posted a staggering one-year return of 29% as of 1 May.

Ruchir Sharma, author of The Rise and Fall of Nations, identified retail investors as the engine pulling what might be called the perpetual bull market-now more than a decade-and-a-half old. "Americans are all in on the market, holding more wealth in stocks than their homes, for the first time," he wrote in the Financial Times. "Retail's share of daily trading in US stocks doubled in the last 15 years to 36%."

The...