Mean reversion is the empirical tendency of a financial time series — a price, spread, or ratio — to return toward its historical mean after deviating from it. A process is mean-reverting if deviations are self-correcting: large moves away from the mean are followed, on average, by moves back toward it.
Mean reversion and momentum are competing forces that tend to dominate at different horizons:
- Very short-term (intraday to 1–2 days) — dominated by mean reversion due to order-flow dynamics and market-making behavior (bid-ask bounce, temporary inventory imbalances).
- Medium-term (1–12 months) — momentum tends to dominate in equities.
- Long-term (3–5+ years) — mean reversion re-emerges as value spreads compress and fundamentals reassert themselves.
Statistically, a mean-reverting process has negative autocorrelation at short lags: a positive return tends to be followed by a negative return. The Ornstein-Uhlenbeck (OU) process is the continuous-time mean-reverting model used in pairs trading and fixed income relative value strategies.