We develop a framework to theoretically and empirically analyze investor competition on financial markets. The classic view assumes that markets are very competitive: if a group of investors changes its behavior, other investors react such that nothing happens in equilibrium. Our framework quantifies the strength of the competitive response. We estimate a demand system of institutional investors in the US stock market accounting for two layers of equilibrium: how investors compete with each other in setting their strategies and how prices adjust to clear asset markets. We find that investors react to the behavior of others in the market: when an investor is surrounded by less aggressive traders she trades more aggressively. This reaction reduces the equilibrium consequences of changes in individual behavior by 50%. However, it also implies that the stock market is far from the competitive ideal. A consequence of this result is that the large increase in passive investing over the last 20 years has led to substantially more inelastic aggregate demand curves for individual stocks, by 15%.