The first essay finds that the market betas of value and small stocks have decreased by about 75% in the second half of the twentieth century. The decline in beta can be related to a long-term improvement in economic conditions that made these companies less risky. The failure to account for...

In the first chapter, I study the impact of statistical arbitrage on equilibrium asset prices. Arbitrageurs have to learn about the long-run behavior of the stock price process. They condition their investment strategy on the observation of price and volume. The learning process of the...