The Adaptive Markets Hypothesis: Market Efficiency from an Evolutionary Perspective
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What they found
Lo proposes a middle ground between efficient markets and behavioral finance. He treats market participants as species competing for profit opportunities: strategies work until enough capital crowds in, then decay, then sometimes return when the crowd leaves. Efficiency is not a fixed state but something that rises and falls with competition, and risk premia change over time as market ecology changes. He illustrates with the changing autocorrelation of U.S. stock returns and the boom-bust pattern of hedge fund strategies.
What you can use
- Edges have life cycles. Expect a strategy that worked for years to decay as it becomes crowded.
- Being adaptable matters more than finding one permanent formula.
- Periods of low volatility and apparent stability are when strategies get overcrowded and fragile.
- Do not treat a strategy's backtest as a property of the market; treat it as a snapshot of past competition.
Caveats
More a framework than a testable theory; critics note it can explain anything after the fact. The empirical illustrations are suggestive rather than formal tests.
Tags: efficiency, adaptive-markets, theory
Summaries are our own reading of the paper, not the authors' words. Educational only, not advice. Discuss it in Book Club.