Stephen W. Salant, Profitable Speculation, Stability and Welfare.
Discussion Paper 1974

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''In 1953, Milton Friedman asserted that profitable speculation increases price stability:

`People who argue that speculation can be destabilizing seldom realize that this is largely equivalent to saying that speculators loose money, since speculation can be destabilizing in general only if speculators sell when the currency is low in price and buy when it is high.' ''
"This remark, repeated in 1971, has generated research by Telser, Farrell, Kemp, Schimmler, and others." ... "Farrell,  Kemp, and Schimmler showed that the Friedman proposition is false unless there are only two periods or linear demand curves with the same slope."

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"As positive economics, Friedman's proposition is wrong. As normative economics, it addresses a question of no significance."