There is a position that the efficient market hypothesis is essentially untestable. That's because purported tests of efficient markets are actually joint tests of two claims
- Market X is efficient
- An efficient markets looks / behaves like Y
A rejection of a market X as not behaving like Y could be because market X is efficient but it is false that efficient markets behave like Y or that X is not an efficient market. The earliest citation I know on this issue is Jensen (1978). There are numerous reasonable examples of how this can occur. Consider violations of the Consumption-CAPM. (Grossman and Shiller (1981)). A rejection of the C-CAPM can occur simply because you have failed to identify the proper stochastic discount factor or market portfolio. Or consider a superficially reasonable assumption that returns should not be predicable if markets are efficient. But if there is aggregate (non-diversifiable) risk and there are limits to the ability to move income through time (like in a Lucas-tree economy) then when aggregate output is relatively low assets will be relatively cheap even if that low output was entirely predictable.
I suspect these problems will perpetually impair consensus on the validity of the efficient market hypothesis. A related but unasked question is "Is it useful to assume that markets are efficient?" On this matter, there does seem to be a strong consensus that:
- Prices in big, liquid markets are often very good and hard to beat
- For models where asset prices or financial wealth are inputs to asking other questions, assuming markets are efficient is a good starting point and often good enough.
All that about untestability said, people try to test market efficiency all the time. Most of these tests fall into the family of "market anomalies". Essentially, someone comes up with an investment portfolio strategy that provides a arbitrage opportunity. Here arbitrage has a precise meaning that differs from common usage in finance. An arbitrage requires no cash outflow now or in the future and there is at least one state of the world in the present or future where it generates strictly positive value. The existence of such an anomaly seems plausible evidence that markets are not efficient. However, in practice, it isn't clear that the famous anomaly papers papers actually demonstrate this. What they more typically demonstrate is that fluctuations in some representative but flawed measure of the market portfolio do not explain all returns and therefore that one might be able to construct zero up-front cost a portfolio with no market risk exposure that nevertheless pays out in some states. But such portfolios almost always have negative outcomes as well as positive ones and so are not strictly speaking arbitrages.
An alternative explaination is that there are other risks that are priced besides the market. That's why the rightfully famous and careful Fama and French (1993) is called Common risk factors in the returns on stocks and bonds. But when we really dig into the weeds of what sorts of "risk factors" generate returns, some don't mesh to any clear source of economic risk. That firms with heavy recession exposures should command higher returns make sense. That firms should outperform in some months and not others is much harder to understand through the lens of risk and some prefer to think of as evidence of market inefficiency.
As for why the efficient market hypothesis might fail, there seem to be a few key stories often combined into "the limits of arbitrage". A few examples follow. Informed traders may lack the capital to take large enough positions so that market prices can reflect their information. Markets may not exist to hedge some risks. If risk factors A and B cannot be traded separately and you have a signal on just A it may be impossible to get risk factor A priced properly. Market participants may be risk adverse and not want concentrated exposures in unusual, under-priced risks, and so not have an incentive to make markets efficient. Transaction costs of trade may prevent the profitable exploitation of information.