"Well if you've got information about a company, or you believe that a company is undervalued, you can go out and buy their stock and you can make some profit on it."

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If you have information that a company is not as good as its stock market valuation, you don't have a way to sell that stock unless you already own it. And so that information doesn't get incorporated in the company's stock price as fast if you don't allow short selling.

— Robert F. Engle