The Grossman-Stiglitz paradox

In general the price system does not reveal all the information about “the true value” of the risky asset …

tumblr_n6vk0tAVwh1rlnhn7o1_500The only way informed traders can earn a return on their activity of information gathering, is if they can use their information to take positions in the market which are “better” than the positions of uninformed traders. “Efficient Markets” theorists have claimed that “at any time prices fully reflect all available information” … If this were so then informed traders could not earn a return on their information.

When the efficient markets hypothesis is true and information is costly, competitive markets break down … As soon as the assumptions of the conventional perfect capital markets model are modified to allow even a slight amount of information imperfection and a slight cost of information, the traditional theory becomes untenable. There cannot be as many securities as states of nature. For if there were, competitive equilibrium would not exist …

Because information is costly, prices cannot perfectly reflect the information which is available, since if it did, those who spent resources to obtain it would receive no compensation. There is a fundamental conflict between the efficiency with which markets spread information and the incentives to acquire information.

Sanford Grossman & Joseph Stiglitz