Most research on the wealth impact of takeover defenses uses event-study methodology, where firms' stock returns are analyzed following the announcement of a new defense. Such studies face the difficulty that new defenses may be driven by contemporaneous conditions at the firm; i.e., adoption of a defense may both change the governance structure and provide a signal of managers' private information about impending takeover bids. Event studies of changes in state takeover laws are mostly immune from this problem, but it is difficult to identify a single date for an event that is preceded by legislative negotiation and followed by judicial uncertainty. For these and other reasons, some authors argue that event-study methodology cannot identify the
impact of governance provisions.