The Grossman–Stiglitz Paradox
Why Perfectly Efficient Markets Cannot Exist
Financial markets are often described as efficient systems in which prices fully reflect all available information. This idea, formalised through the Efficient Market Hypothesis (EMH), suggests that consistently outperforming the market should be extremely difficult, as any new information is rapidly incorporated into asset prices.
However, this concept presents a fundamental contradiction. If markets were perfectly efficient, there would be no incentive for investors to acquire information. Yet without investors actively seeking and analysing information, markets could not remain efficient.
This contradiction is captured by the Grossman–Stiglitz paradox, which demonstrates that perfectly efficient markets cannot exist in equilibrium.
The Core Insight
The paradox, introduced by economists Sanford Grossman and Joseph Stiglitz, is based on a simple but powerful observation:
If prices fully reflect all available information, then gathering information has no value.
In such a scenario, rational investors would have no incentive to incur the costs associated with research, analysis, and data acquisition. However, if no one gathers information, prices cannot reflect that information.
This creates a self-defeating loop:
perfect efficiency removes the incentive to produce information
without information production, efficiency breaks down
As a result, markets must remain at least partially inefficient in order to function.
Information as a Costly Resource
A key component of the paradox is the recognition that information is not free.
Obtaining and analysing information requires:
time
computational resources
expertise
financial cost
Investors who invest in research expect to be compensated for these costs through superior returns. For this to occur, prices must deviate; at least temporarily; from their fully informed values. These deviations create the opportunity for informed investors to earn excess returns.
In this sense, inefficiency is not a flaw in the market, but a necessary condition for its operation.
Informed vs Uninformed Investors
The Grossman–Stiglitz framework distinguishes between:
informed investors, who incur costs to acquire information
uninformed investors, who rely on prices without directly gathering information
Informed investors contribute to price discovery by trading on their insights. Uninformed investors benefit indirectly, as prices incorporate some of this information.
However, prices do not become perfectly informative. They remain only partially efficient, ensuring that informed investors retain an incentive to continue their research.
This balance creates a form of equilibrium in which:
information is reflected in prices, but not completely
inefficiencies persist, but are limited
Implications for Market Behaviour
The paradox has several important implications for how markets function:
Persistent Inefficiency
Markets cannot eliminate all inefficiencies. Some degree of mispricing must remain in order to compensate those who invest in information gathering.
Role of Active Management
Active investors play a critical role in maintaining market efficiency. By conducting research and trading on information, they help incorporate new data into prices. Without their participation, markets would become less informative.
Limits of Passive Strategies
While passive investing benefits from the information embedded in prices, it relies on the presence of active participants to generate that information. If all investors were passive, price discovery would deteriorate.
A Dynamic Equilibrium
The Grossman–Stiglitz paradox suggests that markets exist in a state of dynamic equilibrium.
Efficiency and inefficiency coexist:
prices reflect a significant amount of information
but not enough to eliminate the incentive to acquire more
This balance ensures that markets continue to function, with ongoing interaction between informed and uninformed participants.
Beyond Theory
While the paradox originates in economic theory, its implications are observable in real markets.
Different levels of efficiency can be seen across:
asset classes
market capitalisations
geographic regions
Highly liquid, widely followed markets may exhibit relatively high efficiency, while less covered areas may present greater opportunities for information-based advantage.
Conclusion
The Grossman–Stiglitz paradox highlights a fundamental truth about financial markets: perfect efficiency is impossible.
For markets to function, they must provide incentives for investors to gather and analyse information. This requires the existence of inefficiencies that can be exploited, at least temporarily. Rather than viewing inefficiency as a flaw, the paradox suggests it is an essential feature of the market system.
In this sense, the pursuit of information and the presence of mispricing are not contradictions of market efficiency; they are the mechanisms that sustain it.
