We develop a model to study the efficiency of socially responsible investments (SRI) as a market-based mechanism to control firms' externalities. When responsible investors interact with profit-seeking investors, the former tend to concentrate on a subset of firms in the economy, while excluding others. This concentration of responsible capital can mitigate free-riding and coordination issues in the adoption of green technologies, but it can also create product market power and crowd out the green investments of excluded firms. If the crowding-out dominates, firms' aggregate green investments and welfare are higher without SRI. In equilibrium, responsible capital concentrates the most when concentration is the least desirable.