[We use a laboratory market to investigate the behavior of traders who lack informational advantages and have no exogenous reason to trade. We find that these uninformed traders behave largely as irrational contrarian "noise traders," trading against recent price movements to their own detriment. The uninformed traders provide some benefits to the market: increasing market volume and depth, while reducing bid-ask spreads and the temporary price impact of trades. However, their noise trading also diminishes the ability of market prices to adjust to new information. A securities transaction tax reduces uninformed trader activity, but it reduces informed trader activity by approximately the same amount; as a result, the tax does not alter the impact of noise trading on the informational efficiency of the market.]

[We provide experimental evidence that relaxing margin restrictions to allow more short selling can exacerbate overpricing, even though it reduces equilibrium price levels. This is because smart-money traders initially profit more by front-running optimistic investor sentiment than by disciplining prices. When short selling is not possible, competitive pressures among arbitrageurs rapidly drive prices to the equilibrium. However, the risk of margin calls slows the convergence process, because arbitrageurs who sell short too early face substantial losses if they are unable to synchronize their trades with other arbitrageurs (as in Abreu and Brunnermeier. 2002.]

We report the results of three experiments based on the model of Hong and Stein (1999). Consistent with the model, the results show that when informed traders do not observe prices, uninformed traders generate long-term price reversals by engaging in momentum trade. However, when informed traders also observe prices, uninformed traders generate reversals by engaging in contrarian trading. The results suggest that a dominated information set is sufficient to account for the contrarian behavior observed among individual investors, and that uninformed traders may be responsible for long-term price reversals but play little role in driving short-term momentum.

We use two experiments to test predictions about the positive and negative impacts of allowing analysts to revise their forecasts in light of the consensus forecasts. We find that such mutual observation not only facilitates information aggregation, but also induces free riding, which offsets the benefits of information aggregation unless incentives for accuracy are high. In our second experiment, we find that participants acting as investors anticipate these effects in the consensus and adjust their own forecasts accordingly. Our study demonstrates that the positive and negative effects of mutual observation are more complex than typically portrayed in the debate about analyst independence, and provides a framework that can be used in future research on the interactive nature of public forecasting.

We use a laboratory market to investigate the behavior of traders who lack informational advantages and have no exogenous reason to trade. We find that these uninformed traders behave largely as irrational contrarian “noise traders,” trading against recent price movements to their own detriment. The uninformed traders provide some benefits to the market: increasing market volume and depth, while reducing bid-ask spreads and the temporary price impact of trades. However, their noise trading also diminishes the ability of market prices to adjust to new information. A securities transaction tax reduces uninformed trader activity, but it reduces informed trader activity by approximately the same amount; as a result, the tax does not alter the impact of noise trading on the informational efficiency of the market.

We provide experimental evidence that relaxing margin restrictions to allow more short selling can exacerbate overpricing, even though it reduces equilibrium price levels. This is because smart-money traders initially profit more by front-running optimistic investor sentiment than by disciplining prices. When short selling is not possible, competitive pressures among arbitrageurs rapidly drive prices to the equilibrium. However, the risk of margin calls slows the convergence process, because arbitrageurs who sell short too early face substantial losses if they are unable to synchronize their trades with other arbitrageurs (as in Abreu and Brunnermeier. 2002. Journal of Financial Economics 66(2--3):341--60; 2003. Econometrica 71(1):173--204). The Author 2008. Published by Oxford University Press on behalf of The Society for Financial Studies. All rights reserved. For Permissions, please e-mail: [email protected]., Oxford University Press.

We report the results of three experiments based on the model of Hong and Stein (1999) . Consistent with the model, the results show that when informed traders do not observe prices, uninformed traders generate long‐term price reversals by engaging in momentum trade. However, when informed traders also observe prices, uninformed traders generate reversals by engaging in contrarian trading. The results suggest that a dominated information set is sufficient to account for the contrarian behavior observed among individual investors, and that uninformed traders may be responsible for long‐term price reversals but play little role in driving short‐term momentum.