Since 2002, many firms have been required to alter their board of directors and committees to increase management
monitoring. Kinney and McDaniel (1989) and Chhaochharia and Grinstein (2007) provide empirical
evidence suggesting that investments in corporate governance may differ based on firm size, and that underinvesting
in monitoring may be more pronounced in smaller firms. To further testwhether the benefits of recent
changes in companies' governance mechanisms accrue to smaller firms that have underinvested in governance,
we examine the stock market reaction to changes in board structure over the twenty-four months following the
passage of the Sarbanes–Oxley Act. We construct a new composite measure of board structure and regress buyand-
hold abnormal returns on changes that occur in the Board Structure Index, finding that improvements in corporate
governance quality result in economically significant abnormal returns accruing only to the smaller firms
with weak initial board structures.