The risk and return trade‐off, the cornerstone of modern asset pricing theory, is often of the wrong sign. Our explanation is that high‐beta assets are prone to speculative overpricing. When investors disagree about the stock market's prospects, high‐beta assets are more sensitive to this aggregate disagreement, experience greater divergence of opinion about their payoffs, and are overpriced due to short‐sales constraints. When aggregate disagreement is low, the Security Market Line is upward‐sloping due to risk‐sharing. When it is high, expected returns can actually decrease with beta. We confirm our theory using a measure of disagreement about stock market earnings.

We show that retail trading activity has a positive effect on the volatility of stock returns, which suggests that retail investors behave as noise traders. To identify this effect, we use a reform of the French stock market that raises the relative cost of speculative trading for retail investors. The daily return volatility of the stocks affected by the reform falls by 20 basis points (a quarter of the sample standard deviation of the return volatility) relative to other stocks. For affected stocks, we also find a significant decrease in the magnitude of return reversals and the price impact of trades.

We show that collateral constraints restrict firm entry and postentry growth, using French administrative data and cross‐sectional variation in local house‐price appreciation as shocks to collateral values. We control for local demand shocks by comparing treated homeowners to controls in the same region that do not experience collateral shocks: renters and homeowners with an outstanding mortgage, who (in France) cannot take out a second mortgage. In both comparisons, an increase in collateral value leads to a higher probability of becoming an entrepreneur. Conditional on entry, treated entrepreneurs use more debt, start larger firms, and remain larger in the long run.