Do expectations matter or do asset prices fully reflect economic fundamentals? Do hetero geneous beliefs of millions of different investors cancel out on average due to aggregation, or can optimistic or pessimistic views cluster together and cause prices to deviate from underlying economic fundamentals? These questions have been a matter of heavy debate among economists as well as financial practitioners for many decades already. Keynes, for example, argued that stock prices are not governed by an objective view of 'fundamentals', but by 'what average opinion expects average opinion to be'. In Keynes words "Investment based on genuine long term expectation is so difficult as to be scarcely practicable. He who attempts it must surely lead much more laborious days and run greater risks than he who tries to guess better than the crowd how the crowd will behave; and, given equal intelligence, he may make more disastrous mistakes" (Keynes, 1936, p.157).
In contrast, new classical economists have viewed "market psychology", "investors sentiment" and "trend following speculation" as being irrational and therefore inconsistent with the rational expectations hypothesis (REH) and the efficient market hypothesis (EMH). Friedman, for example, argued that irrational speculative traders would be driven out of the market by rational traders, who would trade against them by taking long opposite positions, thus driving prices back to fundamentals. In Friedman's words: "People who argue that speculation is generally destabilizing seldom realize that this is largely equivalent to saying that speculators lose money, since speculation can be destabilizing in general only if speculators on the average sell when the currency is low in price and buy when it is high" [emphasis added] (Friedman, 1953, p.175).