Mean Reversion

What Is Mean Reversion?

Mean reversion is the idea that prices, returns, and other financial measures tend to move back toward their long-run average over time. When a value strays far above or below its historical norm, the theory holds that it will eventually drift back toward that norm. Analysts use this concept to judge whether an investment has moved too far in one direction.

The idea applies to many things, including individual stock prices, valuation multiples, interest rates, and the price gap between two related assets. It is often discussed as the opposite of momentum, the tendency for prices to keep moving in the same direction.

How It Works

Analysts usually start by measuring how far a value sits from its average. A simple approach compares the current price with a moving average taken over a set number of trading days. A more formal approach uses a standard deviation band around the average to flag readings that are unusually high or low.

Mean reversion strategies assume that an extreme reading is more likely to be followed by a move back toward the average than by a continued move away from it. Some traders buy assets that have fallen well below their average and sell those that have risen well above it. Because the reversal may never arrive, these strategies also need a plan for cases where the move keeps going.

Why It Matters to Investors

Mean reversion offers a useful lens on valuation and risk. An asset trading far above its historical norm may be priced for strong results, while one far below may be out of favor. The concept has real limits, however. Some changes are lasting. A company's business may weaken, an industry may shift, or the underlying economics may change, so the old average may no longer be a reliable guide.

Prices can also stay far from their average for long stretches. This is why a reversal bet can lose money before it pays off, and why the idea is debated among researchers. Some argue that prices move almost unpredictably, a view related to the random walk theory and the efficient market hypothesis. Others point to evidence that certain markets show short-term reversal patterns.