The social value of overreaction to information☆

We study the welfare effects of overreaction to information in markets with asymmetric information as well as the impact of a simple intervention in the form of a tax or a subsidy on trade volume. A large enough level of overreaction is always welfare decreasing: in these situations, introducing a tax can improve welfare. However, a small degree of overreaction can increase welfare. This is because of the interplay of two competing externalities: an information externality, due to the informational role of prices, and a pecuniary externality, due to the allocative role of prices. Depending on the balance of these externalities, a trade volume subsidy may be optimal.