Winners Keep Winning

Momentum investing is based on the empirical observation that stocks that have outperformed recently tend to continue outperforming in the near term, and stocks that have underperformed tend to continue underperforming. It's one of the most robust market anomalies — documented across decades, countries, and asset classes.

The Academic Evidence

The landmark 1993 study by Jegadeesh and Titman found that buying stocks with the best 6-month returns and holding them for 6 months generated significant excess returns. Follow-up studies confirmed the effect across global markets. Momentum isn't a fluke — it's a persistent market inefficiency driven by investor psychology (underreaction to news, then overreaction as trends attract more buyers).

Types of Momentum

Price momentum — Stocks with the strongest relative performance over the past 6-12 months. This is the classic academic definition.
Earnings momentum — Stocks where analysts are raising earnings estimates. Positive estimate revisions tend to be followed by more positive revisions (the "earnings revision cascade").
Relative strength — How a stock performs compared to the broader market. A stock rising 5% while the market falls 3% is showing exceptional relative strength.

Implementing a Momentum Strategy

Define your universe — Start with a broad set (S&P 500, or top 1000 by market cap). Momentum works best with liquid, widely followed stocks.
Rank by momentum — Use 6-month or 12-month total return (excluding the most recent month to avoid short-term reversal effects).
Buy the top quintile — The top 20% of your ranked universe. Hold 15-25 positions for diversification.
Rebalance regularly — Monthly or quarterly. Momentum signals decay — you need to refresh the ranking.
Cut losers quickly — If a stock drops out of the top quintile, sell it. Momentum investors have no loyalty — the exit criteria is math, not emotion.

The Risks of Momentum

Momentum crashes — Periodically, momentum strategies suffer sharp drawdowns (often 20-40%). These typically happen at market turning points, when beaten-down value stocks surge and momentum stocks reverse. The strategy recovers, but the drawdowns are painful.
High turnover — Momentum portfolios trade frequently, which generates short-term capital gains taxes. Best implemented in tax-advantaged accounts.
Whipsaws — In choppy, directionless markets, momentum signals flip frequently, generating losing trades. Momentum works best in trending markets.

Key Takeaways

Momentum is one of the most robust market anomalies — stocks that have done well tend to keep doing well.
Rank by 6-12 month return, buy the top performers, rebalance quarterly, and cut losers ruthlessly.
Momentum experiences periodic crashes (20-40% drawdowns). Manage position sizes accordingly.