Market Efficiency Reloaded: Why Insider Trades do not Reveal Exploitable Information

Several studies have emphasized a slow price adjustment to reported insider trades for Germany. The results presented in this paper, though, show that this is mainly caused by a subset of high arbitrage risk stocks. In fact, the abnormal return difference between the quintiles of stocks with highest and lowest idiosyncratic risk is in the range of 2.99-4.90% over a 20-day interval. These results are robust even in the context of a joint generalized least squares approach. By developing a simple zero-investment arbitrage trading strategy mimicking insider trades, it turns out that such a trading strategy, in most cases, generates significant positive returns as long as transaction costs are neglected. However, the outperformance disappears in all risk quintiles, if bid/ask spreads are taken into account. We conclude that the market's under-reaction to reported insider trades can mainly be explained by the cost of risky arbitrage and is therefore not exploitable. Copyright 2009 The Authors. Journal Compilation Verein für Socialpolitik and Blackwell Publishing Ltd. 2009.