We examine relationship between common institutional ownership and corporate social responsibility (CSR). We find that common institutional ownership is negatively associated with the level of CSR, which supports an anti-competitive view. We conduct a propensity score matching (PSM) analysis and a difference-in-differences (DiD) analysis based on a quasi-natural experiment of financial institution mergers. The results alleviate concerns about endogeneity. Using the DiD setting, we find further support for the anti-competitive view, and can rule out alternative explanations. Additional analyses on investor characteristics show that our results come mainly from common owners with long-term investment horizons or lower social inclination. Moreover, we find that the anti-competitive effect is more pronounced for mature firms, and for firms in industries with lower labor intensity and lower customer sensitivity.