Definition
Alpha decay is the empirical regularity that the excess returns generated by a quantitative trading signal erode over time as the signal becomes more widely known, more capital is deployed against it, and the market adapts. A factor that produced a 12 percent annualised return when first discovered may produce 6 percent five years later and zero after a decade — not because the original research was wrong, but because the market reflects the discovery in prices.
The phenomenon is closely tied to the adaptive-markets hypothesis, which frames financial markets as evolutionary systems in which strategies compete for the same finite pool of mispricing. Once a profitable rule is published, replicated by enough participants, or arbitraged by faster competitors, the conditions that gave it an edge no longer hold.
Why it matters
How it works
Decay has three distinguishable drivers, often confused. Crowding is the mechanical effect of more capital trading the same signal — entries get crowded with similar buyers, exits with similar sellers, and the slippage tax rises until it consumes the edge. Information diffusion is the spread of the underlying insight from a few practitioners to many, through papers, conferences, vendor data products, or hires moving between firms. Regime change is the world itself moving — a signal that exploited a structural feature (a tick-size rule, a settlement quirk, a regulatory gap) can be killed instantly when that feature changes.
The practical defence is a portfolio of signals at different stages of their lifecycle. Quants speak of a research funnel in which raw ideas are tested, promising ones are sized small in production, mature ones are scaled up as conviction grows, and decaying ones are wound down before they bleed capital. Walk-forward backtesting, out-of-sample testing on data the researcher could not have seen, and monitoring of live-versus-backtest divergence are the early-warning systems. When realised performance falls persistently outside the backtest's confidence band, the assumption is decay until proven otherwise.