NEW HAVEN - A speculative bubble is a social epidemic whose contagion is mediated by price movements. News of price increase enriches the early investors, creating word-of-mouth stories about their successes, which stir envy and interest. The excitement then lures more and more people into the market, which causes prices to increase further, attracting yet more people and fueling “new era” stories, and so on, in successive feedback loops as the bubble grows. After the bubble bursts, the same contagion fuels a precipitous collapse, as falling prices cause more and more people to exit the market, and to magnify negative stories about the economy.
But, before we conclude that we should now, after the crisis, pursue policies to rein in the markets, we need to consider the alternative. In fact, speculative bubbles are just one example of social epidemics, which can be even worse in the absence of financial markets. In a speculative bubble, the contagion is amplified by people’s reaction to price movements, but social epidemics do not need markets or prices to get public attention and spread quickly.
Some examples of social epidemics unsupported by any speculative markets can be found in Charles MacKay’s 1841 best seller Memoirs of Extraordinary Popular Delusions and the Madness of Crowds.The book made some historical bubbles famous: the Mississippi bubble 1719-20, the South Sea Company Bubble 1711-20, and the tulip mania of the 1630’s. But the book contained other, non-market, examples as well.
MacKay gave examples, over the centuries, of social epidemics involving belief in alchemists, prophets of Judgment Day, fortune tellers, astrologers, physicians employing magnets, witch hunters, and crusaders. Some of these epidemics had profound economic consequences. The Crusades from the eleventh to the thirteenth century, for example, brought forth what MacKay described as “epidemic frenzy” among would-be crusaders in Europe, accompanied by delusions that God would send armies of saints to fight alongside them. Between one and three million people died in the Crusades.