Jeremy Grantham is the Chairman of the Board of Grantham Mayo Van Otterloo, an American investor well known among institutional investors, but relatively unknown to retail investors. He is regarded as a highly knowledgeable investor in various stock bond and commodity markets. Grantham started one of the world's first index funds in the early 1970s and currently manages approximately $120 billion US. Grantham's quantitative research has revealed reversion to the mean in all bubbles in all commodity, stock and bond markets studied excluding timber.
Ingenious new financial instruments certainly facilitated and exaggerated these weaknesses, but they were not the most potent ingredient in our toxic stew. That honor goes to the economic establishment for building over many decades a belief in rational expectations: reasonable,economically-induced behavior that would always guarantee approximately effi cient markets. In their desire for mathematical order and elegant models, the economic establishment played down the inconveniently large role of bad behavior, career risk management, and flat-out bursts of irrationality.
The dominant economic theorists so valued orderliness and rationality that they actually grew to believe it, and this false conviction became increasingly dangerous. It was why Greenspan and Bernanke were not sure that bubbles – outbursts of serious irrationality – could even exist. It was why Bernanke, who had studied the bubble of 1929, could still not see it as proof of irrationality and could still view the Depression (à la Milton Friedman) as a mere consequence of incredibly bad, easily avoidable policy measures.