Abstract: Although empirical studies show that common shareholding affects corporate conduct and that common horizontal shareholding lessens competition, critics have argued that the law should not take any action until we have clearer proof on the causal mechanisms. I show that we actually have ample proof on causal mechanisms, but that antitrust enforcement should focus on anticompetitive market structures, rather than on causal mechanisms. I debunk claims that every type of causal mechanism that might produce anticompetitive effects is either empirically untested or implausible. I also show that critics are wrong in claiming that common shareholders lack incentives to influence corporations to increase portfolio value by lessening competition. Finally, I show that preventing anticompetitive horizontal shareholding need not restrict diversification or discourage desirable institutional investor influence on corporate conduct.