Momentum

The observed tendency for recent price performance to persist over intermediate time horizons.

Momentum describes the pattern where stocks that have risen recently tend to continue rising, and stocks that have fallen tend to continue falling, over periods of roughly 3 to 12 months. This pattern has been documented across markets, time periods, and asset classes.

The momentum effect contradicts the efficient market hypothesis, which predicts that past returns carry no information about future returns. Yet momentum remains one of the most robust anomalies in finance.

The Momentum Effect

Historical Evidence

• Stocks in the top performance decile have historically outperformed bottom decile stocks by significant margins
• The effect is strongest over 3-12 month horizons
• Momentum exists across equity markets globally
• Similar patterns appear in bonds, commodities, and currencies

Proposed Explanations

Several theories attempt to explain why momentum persists:

• Behavioral biases: Investors may underreact to news initially, then overreact as trends become visible
• Slow information diffusion: News spreads gradually through different investor groups
• Herding: Investors follow trends, reinforcing price movements
• Risk compensation: Momentum stocks may carry higher crash risk, and the return differential may reflect that risk

Measuring Momentum

Price Momentum

• 12-1 month momentum: Return over past year excluding the most recent month (most common academic measure)
• 6-month momentum: Return over prior six months
• Relative strength: Performance versus market or sector benchmark

Technical Indicators

• RSI (Relative Strength Index): Measures speed and magnitude of price changes
• MACD: Compares short and long-term moving averages to identify trend changes
• Stochastic oscillator: Compares closing price to price range over time
• Rate of change: Percentage price change over a specific period

Momentum Strategy Types

Cross-Sectional Momentum

Ranks stocks by recent performance relative to peers. Portfolios are constructed by selecting top performers and avoiding bottom performers, rebalanced periodically.

Time-Series Momentum

Evaluates a stock's own trend. Positions are based on whether a stock's recent returns are positive or negative, applied to individual stocks or portfolios.

Risks and Limitations

Momentum Crashes

• Sudden reversals when trends change direction
• Often coincides with market stress and volatility spikes
• The 2009 momentum crash was particularly severe
• Drawdowns can be rapid and large

Transaction Costs

• High turnover increases trading commissions
• Bid-ask spreads erode returns
• Market impact costs in less liquid stocks

Crowding

When many participants pursue the same momentum signals, the resulting crowding may reduce the effect's magnitude as too many investors chase the same stocks.

Combining Momentum with Other Dimensions

• Quality momentum: Momentum observed in high-quality stocks
• Value momentum: Momentum among stocks with low valuation multiples
• Size momentum: Momentum in small-cap stocks

Momentum describes an observed statistical pattern, not a guarantee of continuation. The same persistence that produces the effect also produces sharp reversals when conditions change. The pattern is well-documented, but the timing and magnitude of reversals are not predictable from the momentum signal itself.