18/2/2013: Short-selling and Markets Volatility

A large number of analysts and policy makers tend to believe that highly leveraged trading activity, especially that linked to HFT, is a significant, even if only partial, driver of markets volatility. The channel through this logic usually works is that in the presence of leverage, speed of positions unwinding in response to unforeseen events increases, thus amplifying volatility.

An interesting study by Harrison Hong, Jeffrey D. Kubik and Tal Fishman, titled "Do arbitrageurs amplify economic shocks?" (Journal of Financial Economics, vol 103 number 3, March 2012, pages 454-470) examined the impact of arbitrageurs' activity on stock performance. Based on quarterly data from 1994 through 2007 for NYSE, Amex, and Nasdaq, share prices were examined over two distinct sub-periods: one day before earnings announcement and one day after the announcement. Medium-term performance was analysed for two days before earnings announcement and 126 days after earnings announcement.

The authors find that:

1. Stock price reaction to earnings news is more severe in heavily shorted stocks than in stock with fewer short positions;
2. Changes in the short ratio and earnings surprises counter-move;
3. Share turnover as a result of large earnings surprises is higher for heavily shorted stocks as consistent with (1) above;
4. Positive earnings surprises push up the valu of heavily shorted shares (as consistent with (1) and (2) above)
5. Following positive earnings announcement, returns are higher (in general) for stocks with heavy shorting positions prior to the announcement since price appreciation post-announcement forces covering of short positions and triggers more demand for shares;
6. Consistent with (5) above, post-positive earnings announcement, previously heavily shorted stocks become better targets for further shorting;

Overall, the study finds that:

• Any earnings surprise in any direction (either positive or negative) leads to a corrective action by (either long or short) investors;
• The above increases price sensitivity to newsflow and thus volatility;
• Trading volume and stock price increase abnormally for heavily shorted stocks;
• The abnormal volatility and volume & price effects are temporary and in the medium terms, prices revert to the mean.