Waddock, Graves, and Gorski (1998)
Waddock, Sandra A., Samuel B. Graves, and Renee Gorski. "Social and Traditional Investment: Stakeholder and Performance Characteristics." The Investment Research Guide to Socially Responsible Investing, The Colloquium on Socially Responsible Investing, 1998.
Used Compustat database to compare the returns of those S&P 500 companies included in the Domini Social Index (N=249) with those excluded from it (N=251). [2018 note: This index is now called the MSCI KLD 400.]. Found that included companies outperformed excluded firms on a 3-year, 5-year, and 10-year basis. The authors do not adjust for risk. This study is the first to note that firms included in the DSI have a higher average number of employees than those which are excluded. This is counterintuitive given the included companies' lower average market capitalization and sales.