Bebchuk, Lucian, Alma Cohen, and Allen Ferrell. "What Matters in Corporate Governance?" Review of Financial Studies, February 2009.

Author's abstract:

We investigate which provisions, among a set of twenty-four governance provisions followed by the Investor Responsibility Research Center (IRRC), are correlated with firm value and stockholder returns. Based on this analysis, we put forward an entrenchment index based on six provisions - four constitutional provisions that prevent a majority of shareholders from having their way (staggered boards, limits to shareholder bylaw amendments, supermajority requirements for mergers, and supermajority requirements for charter amendments), and two takeover readiness provisions that boards put in place to be ready for a hostile takeover (poison pills and golden parachutes). We find that increases in the level of this index are monotonically associated with economically significant reductions in firm valuation, as measured by Tobin's Q. We present suggestive evidence that the entrenching provisions cause lower firm valuation. We also find that firms with higher levels of the entrenchment index were associated with large negative abnormal returns during the 1990-2003 period. Moreover, examining all sub-periods of two or more years within this period, we find that a strategy of buying low entrenchment firms and selling short high entrenchment firms out-performs the market in most such periods and does not under-perform the market even in a single sub-period. Finally, we find that the provisions in our entrenchment index fully drive the correlation, identified by prior work, that the IRRC provisions in the aggregate have with reduced firm value and lower stock returns during the 1990s; we do not find any evidence that the other eighteen IRRC provisions are negatively correlated with either firm value or stock returns during the 1990-2003 period.

Data on the entrenchment index for the period 1990-2007, and a list of over seventy-five studies using our entrenchment index, is available for downloading at Lucian Bebchuk's home page.

SSRN abstract is here.
The full article is available here.

Dr. Bebchuk's data on the Entrenchment index for the 1990-2007 time period is here.