Posnikoff, Judith F. "Divestment From South Africa: They Did Well by Doing Good." Contemporary Economic Policy, January 1997.
This event study finds that stocks in 40 companies divesting from South Africa had positive abnormal returns for two- and three- day periods surrounding the public announcement. Posnikoff argues that "the removal of purchasing restrictions such as those imposed by municipal authorities on firms doing business in South Africa may have increased domestic sales of the divesting firms by more than any loss taken from divestment."
See Teoh, Welch, and Wazzan (1999).