Momentum

Stocks that have risen keep rising — for a while.

Price momentumCross-sectional momentum12-1 momentumUMD (Up Minus Down)WML (Winners Minus Losers)

Typical IS Sharpe

0.6 – 1.0

Typical OOS Sharpe

0.4 – 0.7

Capacity

Mid-cap

Signal decay

~6m half-life

High turnover

Overview

Cross-sectional momentum is one of the most replicated findings in empirical finance. Stocks that have outperformed their peers over the past 2–12 months tend to continue outperforming over the next 1–6 months. Jegadeesh and Titman (1993) documented an annualized return spread of roughly 12% per year between winner and loser deciles in US equities. The effect has been confirmed in international markets, across asset classes, and across time periods extending back to the Victorian era.

Economic Intuition

Three competing explanations dominate the literature. Behavioral theories attribute momentum to investor underreaction: good news is incorporated into prices too slowly, so past winners are still undervalued relative to their fundamental outlook. Overreaction theories argue the opposite — investors chase trends, pushing prices beyond fundamentals, generating short-term continuation followed by long-run reversal. Risk-based explanations posit that momentum portfolios have time-varying exposure to systematic risk factors, earning a premium for bearing crash risk. The 2008–09 momentum crash — one of the worst drawdowns in recorded financial history — lends credence to the crash-risk view.

Out-of-Sample Evidence

Strong OOS survival

Momentum is the factor that most convincingly survives out-of-sample, both in time and across geographies. Post-publication decay is detectable but modest compared to value or size. The main risk is not data-mining but strategy crowding: as momentum becomes widely implemented, the crash risk intensifies and the Sharpe degrades during reversals. The 12-1 specification (skip the most recent month) is standard; its survival across decades and markets makes it a reliable benchmark for any alpha discovery exercise.