What Is Mean Reversion?

The Pull Toward Normal

Many things in markets oscillate around a long-run average rather than drifting forever in one direction. Valuations stretch and compress; volatility spikes and fades; sentiment swings between fear and greed. Mean reversion is the observation that, after a big move away from normal, the odds tilt toward a move back. The crucial caveat is timing: mean reversion says extremes tend to resolve, but not when. A stretched market can stay stretched far longer than expected, which is why reversion is a lens, not a trigger.

Why Mean Reversion Happens

Reversion is not magic; it is the result of opposing forces that strengthen as a move becomes extreme. When a price runs far above its average, it becomes expensive, which attracts sellers and profit-takers and discourages new buyers. When it falls far below, it becomes cheap, which tempts bargain-hunters and deters sellers. The same logic applies to the fundamentals beneath prices: unusually high profit margins invite competition that erodes them, and depressed ones get repaired. Think of it as a rubber band, the further something is stretched from its centre, the greater the tension pulling it back. That tension is what mean reversion describes.

How Traders Spot It

Traders use specific tools to measure how far a price has stretched from its mean. The Relative Strength Index, or RSI, flags a market as "overbought" above 70 and "oversold" below 30. Bollinger Bands plot a moving average with bands a couple of standard deviations above and below it, so a price touching the outer band is statistically stretched. A classic mean-reversion trade "fades" these extremes: buying when a market is oversold and near the lower band, selling when it is overbought and near the upper one, betting on a snap back to the middle. The reading itself is just a measure of tension in the rubber band.

Mean Reversion Versus Trend-Following

Mean reversion has a direct opposite: trend-following, or momentum. The two strategies make contradictory bets about the same move.

Neither is right all the time. They suit different market "regimes", and knowing which regime you are in matters more than which philosophy you prefer.

When It Works, and When It Fails

Mean reversion shines in calm, range-bound, choppy markets, where prices genuinely oscillate around a stable centre and stretched readings reliably snap back. Its great danger is the strong trend. In a powerful bull market, a price can stay "overbought" for months while it keeps climbing, and RSI above 70 is confirmation of momentum, not a signal to sell. Fading a real trend is one of the fastest ways to lose money, because the price may simply never return to the old mean, the mean itself is moving. This is the practical face of the warning that "the market can stay irrational longer than you can stay solvent". Mean reversion is a probability, not a promise, and it is most dangerous exactly when a trend feels most extreme.

Mean Reversion and Sentiment

Sentiment is strongly mean-reverting, which is the whole basis for watching a Fear and Greed Index. Unlike a price, which can in theory trend upward forever, a sentiment gauge is bounded, it cannot exceed 100 or fall below 0, and emotion physically cannot stay at fever pitch indefinitely. Extreme Fear and Extreme Greed are stretched conditions that historically do not persist, which is why contrarians treat them as moments to pay attention. The same caveat applies, though: sentiment reverts eventually, but the gauge cannot tell you the day, so the extreme is a lens on the odds, never a trigger by itself.