Bagnoli and Watts (2003)
Bagnoli, Marc and Susan Watts. "Selling to socially responsible consumers: Competition and the private provision of public goods." Journal of Economics and Management Strategy (12). 2003.
An econometric study cited by Siegel and Vitaliano (2006), who say "a key finding...is that when the market for the private good is more competitive, firms are more likely to be socially responsible."