What is Momentum Trading? The Physics of Trend Persistence

Momentum trading is the strategy of buying winners and selling losers. Discover the mathematical basis for trend persistence and the dual-momentum framework.

Backtesting 20 years of futures data for 14 hours revealed why Momentum Trading remains the most persistent anomaly in finance despite being the literal opposite of mean reversion. Momentum is the empirical observation that assets which have performed well in the recent past tend to continue performing well over the short to medium term.

Mathematically, momentum is the first derivative of price with respect to time: $v = \frac{dP}{dt}$. In a quantitative framework, we often measure this using 'Relative Strength' (comparing an asset to its peers) or 'Absolute Momentum' (comparing an asset to its own past performance). A common metric is the 12-1 momentum, which calculates the total return over the last 12 months, excluding the most recent month to avoid short-term reversal effects.

Why does momentum work? It is primarily driven by three factors: underreaction to news, herding behavior, and the 'disposition effect' (investors selling winners too early and holding losers too long). As more participants notice the trend, they pile in, creating a self-fulfilling prophecy until the trend reaches an exhaustion point. This contradicts the Efficient Market Hypothesis, which predicts immediate price adjustment.

1. Time-Series Momentum: Betting on an asset's own trend. If $R_{t-12, t-1} > 0$, stay long. 2. Cross-Sectional Momentum: Ranking a universe of stocks and buying the top decile while shorting the bottom decile. 3. Dual Momentum: Combining both to ensure you are in the best-performing assets only when the broad market trend is positive.

A robust momentum signal requires smoothing to avoid being stopped out by noise. We often use a series of Exponential Moving Averages (EMA).