Carter, Simkins, and Simpson (2003)

Carter, David A., Betty J. Simkins, and W. Gary Simpson. "Corporate Governance, Board Diversity, and Firm Value." The Financial Review, February 2003.

Studies the effects of board diversity on firm valuation, focusing on 638 publicly traded members of the Fortune 1000. Finds that diversity (representation of women and/or minorities on the board) is a positive factor in Price/Book ratio. Board data is from 1997. Regressions use Compustat data (presumably also from 1997, though this is not stated) and adjust for industry, total assets, board size, return on assets, and insider ownership %. No control for R&D intensity, however. Finds that woman/minority representation (as a % of board) rises as firm size rises, but decreases as insider board membership increases. Also finds statistically significant negative impact on firm value when CEO and board chairman are the same person.

Link:  https://onlinelibrary.wiley.com/doi/abs/10.1111/1540-6288.00034