In this Article, we examine the causal mechanisms that might link common ownership to anticompetitive effects. We consider whether the current empirical evidence supports the existence of these mechanisms and whether institutional investors would plausibly employ them. Our main conclusion is that most proposed mechanisms either lack significant empirical support or else are implausible. Notably, some widely discussed mechanisms—for example, cartel facilitation and passive failures to encourage competition among portfolio firms—are not empirically tested. Moreover, institutional investors’ incentives to increase portfolio value are weak, reducing the likelihood that these investors will pursue mechanisms that carry significant reputational or legal risks. We find, however, that a different mechanism, which we call “selective omission,” is both consistent with the evidence and plausibly employed by institutional investors. Looking ahead, our analysis suggests paths for future research and provides a guide for further investigation into how common owners and firms may interact to produce anticompetitive effects.
