Mean Reversion Explained

Prices and valuations tend to snap back toward their long-run average after stretching too far in either direction, until, in some cases, they don't.

The pull back toward average

Mean reversion is the observation that many market variables — valuation multiples, volatility, spreads between related assets, even certain economic indicators — tend to gravitate back toward a long-term average after moving to an extreme. A stock trading at an unusually rich multiple relative to its own history, or a market showing unusually low volatility for an extended stretch, tends over time to drift back toward more typical levels, rather than staying at the extreme indefinitely.

The intuition underneath this is straightforward: extremes are, by definition, unusual, and many of the forces that create them, temporary shifts in sentiment, short-term supply-demand imbalances, a burst of unsustainable enthusiasm or panic, are themselves temporary. As the specific conditions that pushed a variable to an extreme fade, the variable tends to fade back with them.

Where mean reversion works well

Valuation multiples are a classic domain for mean reversion — a sector trading at a significant premium or discount to its own long-run average multiple, absent a genuine structural change in its growth or profitability outlook, has historically tended to converge back toward that average over multi-year periods. Volatility is another strong case: periods of unusually low or unusually high volatility have rarely persisted indefinitely, because both extremes tend to be self-correcting.

Interest rate spreads, currency valuations relative to purchasing power measures, and commodity prices relative to production costs also show reasonably reliable mean-reverting tendencies over long horizons, because they're anchored to real economic constraints that eventually reassert themselves.

Where it fails

Mean reversion breaks down specifically when the mean itself is moving, when there's a genuine structural or secular shift underway rather than a temporary extreme around a stable average. A company or sector experiencing a real, durable improvement in its growth rate or margin structure can trade at what looks like an extreme valuation multiple for years, because the old average no longer describes the business the multiple should be measured against.

This is the central judgment call embedded in every mean-reversion trade: is the current extreme a temporary deviation from a stable underlying average, or early evidence that the average itself has shifted? Getting that distinction wrong is the most common way mean-reversion strategies lose money.

The contrast with momentum

Mean reversion and momentum describe genuinely opposite behaviors — one bets a stretched move corrects, the other bets a trend continues — and both are demonstrably true depending on the asset, timeframe, and regime being examined. Short-horizon price moves often show momentum characteristics, while longer-horizon valuation extremes more often show mean-reverting characteristics. Reconciling the two isn't a contradiction to resolve so much as a reminder that the right framework depends heavily on the specific variable and time horizon in question.