EMH, developed by economist Eugene Fama in the 1960s (earning him a Nobel Prize in 2013), is simultaneously the most widely accepted and most contested idea in finance. The professional investment industry is built on the premise that some managers can beat the market. EMH says that in aggregate, they can't — and the evidence largely supports this.
The three forms of efficiency
| Form | Prices reflect | Implication |
|---|---|---|
| Weak | All past prices and trading data | Technical analysis cannot work |
| Semi-strong | All public information | Fundamental analysis cannot give an edge |
| Strong | All information, including private | Even insider information is priced in |
Most academics accept weak-form efficiency. Semi-strong is debated. Strong form is widely rejected (insider trading laws exist because insiders demonstrably profit from their information).
The evidence for EMH
The data is striking: fewer than 20% of actively managed US equity funds beat their benchmark over 10 years, and fewer than 10% over 20 years. After accounting for fees, the numbers are worse. Mutual fund managers who outperform in one decade have no better than random odds of outperforming in the next. If skill existed consistently, we'd see persistence of performance — and we largely don't.
The evidence against EMH
Markets have exhibited persistent "anomalies" that EMH cannot fully explain: the value premium (cheap stocks outperform expensive ones over time), the momentum effect (recent winners keep winning), the small-cap premium, and seasonal effects. Behavioural economists argue these anomalies arise from systematic human irrationality — overconfidence, loss aversion, herding. The fact that some investors (Buffett, Lynch) have produced returns that are statistically extremely unlikely under EMH is also uncomfortable for the hypothesis.
The market is mostly efficient — but not perfectly so. The question is whether the inefficiencies are large enough, and persistent enough, for the costs of trying to exploit them to be worth it.
What this means for you
The practical takeaway from EMH — even if you don't fully accept it — is that beating the market is very hard, and fees eat away at performance. For most investors most of the time, low-cost index funds outperform actively managed alternatives over the long run. If you want to pursue active strategies, concentrate on less-analysed areas (small-cap, emerging markets) where markets may be less efficient than for the largest, most-watched companies.