The Burden of Proof Has Shifted
If markets are even semi-strong efficient, then analysing publicly available information to find mispriced securities is — in aggregate — a zero-sum game before costs and a negative-sum game after them. This does not mean every active manager underperforms every year. It means that the burden of proof lies squarely with active strategies to demonstrate genuine, persistent alpha after fees.
The evidence has largely failed to discharge that burden. Study after study, across geographies, time periods, and asset classes, shows that most active managers underperform their passive benchmark over horizons of ten years or more. The minority who do outperform are not reliably identifiable in advance.
Past Performance Does Not Predict Future Performance
This is perhaps the most practically important implication. Investors instinctively use recent outperformance as the primary criterion for selecting active managers. The research is clear: past alpha is a poor predictor of future alpha. A fund's top-quartile performance in one period is barely more informative about the next period than a coin flip. The main exception — negative persistence — is more robust: poor performers are somewhat more likely to continue underperforming (partly because high fees are a persistent drag).
The Two-Level Problem
Even if a minority of active managers do possess genuine skill, investors face a second problem: identifying them in advance. Past performance is noisy. The manager who outperformed last decade may have benefited from style tailwinds (a value run, a growth run) that are now mean-reverting. Distinguishing luck from skill requires many decades of data — far more than most managers have on record.
The Joint Hypothesis Problem
A subtle technical point matters here. Any test of market efficiency is simultaneously a test of the asset pricing model used as the benchmark. If a strategy appears to generate alpha relative to the CAPM, this might reflect genuine inefficiency — or it might mean the CAPM is an incomplete model of risk (which Fama himself later demonstrated with his multi-factor models). Anomalies can persist in the literature for years before researchers establish whether they reflect true inefficiency or simply missing risk factors.
The Practical Conclusion
The practical implication for most investors is not that active management is always wrong, but that the default should be passive. An investor considering an active strategy should ask: what specific, durable edge does this manager possess that is not already priced in, and is the fee justified by the expected alpha net of costs and taxes? In most cases, a rigorous answer to that question leads back to low-cost index funds.