It is commonly believed that prices in secondary financial markets play an important allocational role because they contain information that facilitates the efficient allocation of resources. This paper identifies a limitation inherent in this role of prices. It shows that the presence of a feedback effect from the financial market to the real value of a firm creates an incentive for an uninformed trader to sell the firm's stock. When this happens the informativeness of the stock price decreases, and the beneficial allocational role of the financial market weakens. The trader profits from this trading strategy, partly because his trading distorts the firm's investment. We therefore refer to this strategy as "manipulation". We show that trading without information is profitable only with sell orders, driving a wedge between the allocational implications of buyer and seller initiated speculation, and providing justification for restrictions on short sales. Copyright 2008 The Review of Economic Studies Limited.

Capital Markets and the Economy, and the Bank of Sweden Conference on "Beliefs and Business Cycles" for helpful comments. Itay Goldstein gratefully acknowledges financial support from the Rodney White Center. Alexander Guembel thanks the Paul Woolley Research Initiative for funding and the University of Mannheim, where part of this research was carried out, for their kind hospitality.

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