If competitive equilibrium is defined as a
situation in which prices are such that all
arbitrage profits are eliminated, is it possible
that a competitive economy always be in
equilibrium? Clearly not, for then those who
arbitrage make no (private) return from
their (privately) costly activity. Hence the
assumptions that all markets, including that
for information, are always in equilibrium
and always perfectly arbitraged are inconsistent when arbitrage is costly.
We propose here a model in which there
is an equilibrium degree of disequilibrium:
prices reflect the information of informed
individuals (arbitrageurs) but only partially,
so that those who expend resources to obtain information do receive compensation.
How informative the price system is depends on the number of individuals who are
informed; but the number of individuals
who are informed is itself an endogenous
variable in the model.
The model is the simplest one in which
prices perform a well-articulated role in conveying information from the informed to the
uninformed. When informed individuals observe information that the return to a security is going to be high, they bid its price up,
and conversely when they observe information that the return is going to be low. Thus
the price system makes publicly available
the information obtained by informed individuals to the uniformed. In general, how ever, it does this imperfectly; this is perhaps
lucky, for were it to do it perfectly, an
equilibrium would not exist.
In the introduction, we shall discuss the
general methodology and present some conjectures concerning certain properties of the
equilibrium. The remaining analytic sections
of the paper are devoted to analyzing in
detail an important example of our general
model, in which our conjectures concerning
the nature of the equilibrium can be shown
to be correct. We conclude with a discussion
of the implications of our approach and
results, with particular emphasis on the relationship of our results to the literature on
"efficient capital markets.