What Works on Wall Street Ch. 3: Why Momentum Matters

"Investors should buy stocks that have already started rising. Stocks performing well over the past six to twelve months tend to keep performing well in the near future." — James O'Shaughnessy

Investment Context

Value works over long horizons but carries a practical problem: cheap stocks can stay cheap for a very long time. A deeply discounted stock nobody cares about may do nothing for five years.

To address this, O'Shaughnessy tested a second factor: momentum — the tendency of winners to keep winning and losers to keep losing over six to twelve months.

The Wall Street Translation

1. Why Momentum Exists

The standard explanation is behavioural: people react too slowly to new information (anchoring bias). When a company reports far better results than expected the price rises, but investors anchored to the old price do not immediately push it to fair value. The market takes months to absorb the new reality, producing a sustained trend.

2. Relative Strength

O'Shaughnessy measured momentum through relative strength — how much a stock rose over six months against the rest of the market. Buying the highest relative strength group materially improved returns.

3. Momentum as the Catalyst Value Lacks

This is the book's most important combination insight: value tells you what to buy; momentum tells you when the market is beginning to agree. It supplies the time dimension value investing is missing.

It speaks directly to The Little Book of Value Investing Chapter 4 elsewhere in this library: Browne concedes catalysts cannot be predicted, leaving you to buy cheap and wait. O'Shaughnessy's answer is that you need not predict the catalyst — you can wait for price itself to confirm it has arrived.

4. Momentum's Price: Crash Risk

The dark side deserves stating. Momentum strategies suffer severe drawdowns at sharp market reversals — in the rebound after March 2009 momentum lagged badly while holding the prior period's winners, and similar episodes recurred in 2008 and other crises.

Momentum's return is not free; it is paid for in tail risk. This connects directly to the framework in Antifragile elsewhere in this library: a strategy that wins slightly most of the time and loses enormously on occasion carries far more real risk than its volatility suggests.

Actionable Trading Rules

1. Do not catch a falling knife: Never buy purely on cheapness while a stock sits in a clear downtrend. Wait for the price to stabilise.
2. Wait for trend confirmation: Put undervalued names on a watchlist and buy only once a six-month chart shows a clear uptrend.
3. Understand the tail risk you are taking: If you use momentum, accept in advance that it concentrates its losses at violent reversals, and size positions accordingly.

Relevance to a Retirement Portfolio

Momentum warrants particular caution for retirees, for two reasons.

First, taxes: momentum turns over frequently, and in a taxable account short-term gains taxed as ordinary income can consume most of the excess return. Second, the timing of its tail risk: momentum crashes typically occur during violent market reversals — precisely when a retirement portfolio is most fragile. If you are withdrawing at the same time, those losses are locked in permanently.

So if you use momentum at all, confine it strictly to tax-deferred accounts and to a small share of total assets.