Market Efficiency and Insider Trading: New Evidence

That corporate insiders earn profits from stock trading does not surprise most financial economists, but that outsiders can earn abnormal returns by using pub licly-available, insider-trading data constitutes a serious exception to stock-market efficiency. The authors show that this anomaly conti nues to exist despite the publication of studies attesting to its exi stence. They suggest that the anomalous profits to outsiders are a ma nifestation of the size and earnings/price ratio effects. Controlling for these factors reduces outsider profits by half: the additional a ssumption of a 2 percent transactions cost makes outsider profits zer o or negative. Insider profits, after an assumed 2 percent transactio ns cost, are a moderate 3 percent per annum for annual holding period s. Copyright 1988 by the University of Chicago.

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