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Potential Pilot Problems: Treatment Spillovers In Financial Regulatory Experiments, Ekkehart Boehmer, Charles Jones, Xiaoyan Zhang

Research Collection Lee Kong Chian School Of Business

In analyzing regulatory experiments, a fundamental assumption is that the control group is unaffected. However, in many settings, this assumption may not hold. Generally, the total effect of a regulatory change consists of direct and indirect effects, but the standard difference-in-difference approach measures only direct effects. We apply our methods to the 2007 repeal of the uptick rule by the SEC. The indirect effects are substantial, because unlike the 2005 partial repeal, total repeal enables aggressive portfolio shorting. In particular, we find that short sellers become much more aggressive across the board, and shorting activity increases, even in control stocks …