"Buy low, sell high" is the most repeated advice in investing. Momentum proposes almost the opposite: buy what has already risen, because it tends to keep rising for a while. It sounds like a contradiction, and yet it is one of the most studied patterns in the stock market. This guide explains what it is, why it might exist, how it is measured and, above all, how it can go wrong.
What momentum is
Momentum is the tendency of stocks that have performed well over recent months to keep performing relatively well for a while longer, and of those that have performed badly to keep lagging.
The key words are relatively and for a while. Momentum does not say that a rising stock will rise for ever, nor that it will rise every month. It says that, on average and across many stocks, recent winners have tended to beat recent losers over the following months, before the effect fades.
There are two ways of looking at it:
- Relative momentum compares stocks with each other: it buys those that have risen most compared with the rest.
- Absolute momentum (or trend following) looks at each stock on its own: it buys if the stock's own trend is up, whatever the others do.
Why it might exist
If momentum were simple and obvious, the market should have eliminated it long ago. That it has been observed for decades and in many markets has several explanations, none definitive:
1. Slow reaction to news. Investors take time to absorb good or bad news. A company that improves does not reprice in one day; it does so over weeks or months, as more investors become convinced.
2. Herd behaviour. Rises attract attention, attention attracts buyers and buyers push the price up again. At some point the movement goes beyond what the news justified.
3. Money flows. Funds that receive money tend to buy what they already hold, and many managers avoid being seen with the year's losers in their portfolio.
4. Compensation for risk. Part of the research argues that momentum is a reward for bearing a specific risk: the risk of the sharp reversals we will see below.
What matters for an investor is not which explanation is right, but that the effect is not guaranteed and can disappear for long periods.
How momentum is measured
There are many ways, and all of them try to answer the same question: is this stock moving up with strength?
Return over the last 12 months, skipping the last one
The most common academic measure takes the return of the last twelve months excluding the most recent month. The last month is excluded because, over very short periods, prices tend to do the opposite: they often give back part of the latest move.
Distance to the 52-week high
A stock trading close to its highest price of the last year is, by definition, in a strong position. Many investors watch this measure because it is simple and does not depend on a starting point.
Moving averages
A price above its 50-day and 200-day averages, with the 50 above the 200, describes an orderly uptrend. We explain it in The 50 and 200-Day Moving Averages.
Strength of the trend
Indicators such as the ADX do not measure direction but intensity: whether a trend, up or down, is strong or weak. We explain it in The ADX.
Breakouts with volume
A price that breaks above a level where it had stopped several times, with unusually high trading volume, suggests that many buyers have arrived at once.
The risks: why momentum hurts when it turns
Momentum is not a free lunch. These are its main dangers:
1. Sharp reversals
The worst periods for momentum usually come right after a big market fall, when the market rebounds hard. In those moments, the stocks that had fallen most (the ones momentum avoided) recover fastest, while the ones that had held up best lag behind. A momentum strategy can lose in a few weeks what it had gained in months.
A well-known example is the spring of 2009: after the financial crisis, the most punished stocks rebounded violently and momentum strategies suffered one of their worst episodes.
2. You always buy after a rise
By definition, momentum enters when the price has already gone up. If the move ends just after you enter, you bought near the top. It is uncomfortable and it happens often.
3. Sideways markets
When the market moves without direction, apparent trends start and fail one after another. Each one can produce a small loss, and many small losses add up.
4. Costs
Momentum strategies tend to change positions often. Each change pays commissions, spread and, where applicable, taxes. We discuss how costs eat into a strategy in Backtesting and Its Traps.
5. Crowded trades
When many investors follow the same stocks for the same reason, the exit door is narrow. A piece of bad news can make everyone try to sell at the same time.
How to manage those risks
- Decide the exit before entering. A trend can end at any moment; the stop and the conditions for leaving are part of the plan, not an improvisation. See What a Stop Loss Is.
- Diversify. Momentum works, when it does, as an average across many stocks. A single position can always be the exception.
- Accept that winners need time. Cutting every position at the first pullback destroys the logic of the method.
- Look at the market context. The same signal does not behave the same way in every market situation.
Momentum at InbestPlanners
Impulse is our trend-following method. An Impulse signal looks for a stock that is already moving up with strength, from two rules: a strong, well-ordered trend, or a breakout with unusually high volume. Each signal publishes, before it opens, a buy price, a target of +10% on that price, a maximum duration of 125 sessions and its stops: a volatility stop during the first 20 sessions, never further than -15%, a fixed floor at -20% and a trailing stop that starts working from +3%.
Next to each new signal we show the market context, measured separately for Impulse in each market and each market state, labelled according to how sure the measurement is, including "no signal" when the data cannot tell. It describes the past; it is not a forecast.
Outside the signals, the technical ranking orders the companies we cover by a technical score that includes a momentum block with three of the measures above: the return over the last twelve months skipping the last one, the position within the 52-week range and the RSI. The 360 Analysis shows them for each company. If you want to compare momentum with the opposite idea, read Mean Reversion vs Trend Following. The full rules are on the methodology page.
In short
- Momentum is the tendency of recent winners to keep doing relatively well for a while.
- It may come from slow reactions to news, herd behaviour and money flows; none of it is guaranteed.
- It is measured with past returns, distance to the 52-week high, moving averages and breakouts with volume.
- Its main risk is the sharp reversal after a big fall, when the market rebounds and the leaders lag.
- Decide the exit before entering, diversify and accept that you will often buy after a rise.
This content is for educational purposes only and does not constitute financial advice. Past performance does not guarantee future results.