Wokutch, Richard E. "The End of South African Sanctions, Institutional Ownership, and the Stock Price Performance of Boycotted Firms: Evidence of the Impact of Social/Ethical Investing." Working Paper, 1998.
This event study finds that the stock market returns for boycotted firms were "significantly greater than the returns for the market as a whole during the period immediately following the end of sanctions," and that institutional ownership of those stocks increased significantly during that time. Includes an analysis of signatories to the Statement of Principles (nee "The Sullivan Principles") which finds that "signatories significantly outperformed non-signatories during the same period."
But also see Teoh, Welch, and Wazzan (1999).