We model a market populated by two groups of boundedly rational agents: "newswatchers" and "momentum traders." Each newswatcher observes some private information, but fails to extract other newswatchers' information from prices. If information diffuses gradually across the population, prices underreact in the short run. The underreaction means that the momentum traders can profit by trendchasing. However, if they can only implement simple~i.e., univariate! strategies, their attempts at arbitrage must inevitably lead to overreaction at long horizons. In addition to providing a unified account of under-and overreactions, the model generates several other distinctive implications.

Various theories have been proposed to explain momentum in stock returns. We test the gradual-information-diffusion model of Hong and Stein (1999) and establish three key results. First, once one moves past the very smallest stocks, the profitability of momentum strategies declines sharply with firm size. Second, holding size fixed, momentum strategies work better among stocks with low analyst coverage. Finally, the effect of analyst coverage is greater for stocks that are past losers than for past winners. These findings are consistent with the hypothesis that firm-specific information, especially negative information, diffuses only gradually across the investing public.SEVERAL RECENT PAPERS HAVE DOCUMENTED that, a t medium-term horizons ranging from three to 12 months, stock returns exhibit momentum-that is, past winners continue to perform well, and past losers continue to perform poorly. For example, Jegadeesh and Titman (1993), using a U.S. sample of NYSE/ AMEX stocks over the period from 1965 to 1989, find that a strategy that buys past six-month winners (stocks in the top performance decile) and shorts past six-month losers (stocks in the bottom performance decile) earns approximately one percent per month over the subsequent six months. Not only is this an economically interesting magnitude, but the result also appears to be robust: Rouwenhorst (1998) obtains very similar numbers in a sample of 12 European countries over the period from 1980 to 1995.l * Hong is from the Stanford Business School, Lim is from Goldman Sachs, and Stein is from the MIT Sloan School of Management and the National Bureau of Economic Research. This research is supported by the National Science Foundation and the Finance Research Center at MIT. We are grateful to Joseph Chen for research assistance and to Ken French, Paul Pfleiderer, Geert Rouwenhorst, David Scharfstein, Ken Singleton, Rene Stulz, three anonymous referees, and seminar participants at MIT, Yale, UCLA, Berkeley, Stanford, Illinois, the Norwegian School of Management, and the Stockholm School of Economics for helpful comments and suggestions. Data on analyst coverage were provided by I/B/E/S Inc. under a program to encourage academic research. Thanks also to Lisa Meulbroek for sharing the data on options listings.' Rouwenhorst (1997) finds that momentum strategies also earn significant profits on average in a sample of 20 emerging markets. See Haugen and Baker (1996) for confirmatory evidence from the United States and several European countries.

We provide evidence for the effects of social norms on markets by studying "sin" stocks-publicly traded companies involved in producing alcohol, tobacco, and gaming. We hypothesize that there is a societal norm against funding operations that promote vice and that some investors, particularly institutions subject to norms, pay a financial cost in abstaining from these stocks. Consistent with this hypothesis, we find that sin stocks are less held by normconstrained institutions such as pension plans as compared to mutual or hedge funds that are natural arbitrageurs, and they receive less coverage from analysts than stocks of otherwise comparable characteristics. Sin stocks also have higher expected returns than otherwise comparable stocks, consistent with them being neglected by norm-constrained investors and facing greater litigation risk heightened by social norms. Evidence from corporate financing decisions and time variation in norms for tobacco also suggests that norms affect stock prices and returns.

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We examine security analysts'career concerns by relating their earnings forecasts to job separations. Relatively accurate forecasters are more likely to experience favorable career outcomes like moving up to a high-status brokerage house. Controlling for accuracy, analysts who are optimistic relative to the consensus are more likely to experience favorable job separations. For analysts who cover stocks underwritten by their houses, job separations depend less on accuracy and more on optimism. Job separations were less sensitive to accuracy and more sensitive to optimism during the recent stock market mania. Brokerage houses apparently reward optimistic analysts who promote stocks.

We investigate the effect of scale on performance in the active money management industry. We first document that fund returns, both before and after fees and expenses, decline with lagged fund size, even after accounting for various performance benchmarks. We then explore a number of potential explanations for this relationship. This association is most pronounced among funds that have to invest in small and illiquid stocks, suggesting that these adverse scale effects are related to liquidity. Controlling for its size, a fund's return does not deteriorate with the size of the family that it belongs to, indicating that scale need not be bad for performance depending on how the fund is organized. Finally, using data on whether funds are solo-managed or team-managed and the composition of fund investments, we explore the idea that scale erodes fund performance because of the interaction of liquidity and organizational diseconomies.

We investigate the idea that stock-market participation is influenced by social interaction. We build a simple model in which any given "social" investor finds it more attractive to invest in the market when the participation rate among his peers is higher. The model predicts higher participation rates among social investors than among "nonsocials". It also admits the possibility of multiple social equilibria. We then test the theory using data from the Health and Retirement Study. Social households-defined as those who interact with their neighbors, or who attend church-are indeed substantially more likely to invest in the stock market than non-social households, controlling for other factors like wealth, race, education and risk tolerance. Moreover, consistent with a peereffects story, the impact of sociability is stronger in states where stock-market participation rates are higher.