What is Mean reversion?

Mean reversion is the idea that something which has moved unusually far from its long-run average tends, over time, to move back towards that average. The "mean" is the average. In investing the idea is applied to share prices, to valuations such as the price-to-earnings ratio (a company's share price divided by its profit per share), to interest rates and to the returns of whole asset classes. A market that looks very expensive compared with its own history, or a run of unusually strong years, is often followed by more ordinary conditions.

Here is a hypothetical example. Say a stock market's price-to-earnings ratio has averaged 16 over many decades and today stands at 24. Suppose company earnings grow 5% a year for ten years. Compounded, that is 1.05 multiplied by itself ten times, or about 1.63, so earnings rise by roughly 63%. If the ratio stays at 24, prices rise by the same 63%. If instead the ratio drifts back to 16, the price change is 1.63 × (16 ÷ 24) = 1.63 × 0.67, or about 1.09. That is a gain of only about 9% over the whole decade, under 1% a year before dividends. The return of the valuation to its average absorbed most of the earnings growth.

The main limit is that mean reversion describes a tendency, not a timetable. Prices and valuations can stay far from their average for many years, and the average itself can shift as interest rates, accounting rules or the make-up of an index change. It also sits alongside momentum, the tendency for recent winners to keep rising over shorter periods, and the two effects can operate over different time frames. A common misunderstanding is that anything which has fallen must bounce back. Individual companies can fail outright, and loss arithmetic is unforgiving: an 80% loss needs a 400% gain to recover. The Nasdaq-100 fell 83% from 2000 to 2002, a reminder of how deep declines can go.

For a private investor building a long-term portfolio, mean reversion is mainly a tool for setting expectations. The S&P 500 returned 15.5% a year over the 10 years to June 2026, against 10.7% a year since January 1993. Investors aware of mean reversion tend to treat a strong recent decade as unusual rather than as a baseline to extrapolate, without reading it as a signal of what comes next. The idea also underpins valuation measures such as Robert Shiller's CAPE ratio and helps explain why rebalancing, selling some of what has risen and buying what has lagged, can keep a portfolio close to its intended mix.

By Dr. Harry Hamann

What does mean reversion mean in plain words?
It means that a price, return or valuation that has moved far above or below its long-run average tends to drift back towards that average over time, though not on any predictable schedule.

Is mean reversion the opposite of momentum?
They pull in different directions. Momentum is the tendency for recent winners to keep winning over shorter periods, while mean reversion describes extremes fading over longer periods. Research finds evidence of both, typically over different time frames.

Does mean reversion mean a fallen stock will recover?
No. Mean reversion is a tendency seen in broad markets and valuations, not a promise for any single asset. Individual companies can decline permanently or go bankrupt, and an 80% loss needs a 400% gain to recover.