Mean Reversion
ES: Reversión a la Media PT: Reversão à Média
The tendency of certain variables to return toward their historical average: where the phenomenon is real, where it is a dangerous illusion, and how it is traded with options.
What reverts and what does not
Mean reversion is the property by which a variable that moves away from its historical average tends to return toward it. The critical distinction — and the one separating winners from the ruined — is that not every financial variable reverts. Volatility reverts strongly and is well documented: periods of calm and agitation alternate, and extreme spikes dissipate. Spreads between related assets revert as long as the economic link binding them holds. The price of an individual stock, by contrast, does not revert reliably: a company can fall 80% and keep falling to zero, because no law obliges a price to return to where it was.
Volatility reversion, the exploitable one
Volatility is where the phenomenon is most robust, and not by accident: it has an economic mechanism behind it. Volatility spikes arise from uncertainty, and uncertainty resolves — news gets published, crises get managed, participants adapt — so the level of agitation tends to return toward its average. On top of that, implied volatility persistently trades above the volatility that subsequently materialises, a gap known as the variance risk premium. The combination of both effects is the foundation of all premium-selling: you do not sell because price will stand still, but because the market usually overpays for insurance.
The trap of applying it to price
The most expensive error is transferring volatility intuition to the price of a specific asset. "It has fallen a lot, it has to bounce" is not statistical reasoning but a fallacy: each new session is independent of the prior path, and a 50% decline can be the start of another 50%. The operational version of this trap is averaging down into a losing position, which works many times — which is why it gets reinforced as a habit — until the time it meets a genuinely deteriorating company and erases accumulated capital. The tell that you are making this error is simple: if your only reason for holding or adding is that it has fallen, you do not have a thesis, you have a hope.
How to identify a reasonable candidate
A defensible reversion trade needs three things. First, an economic mechanism explaining why the variable should return: in volatility, the resolution of uncertainty; in a spread between two assets, the real link binding them. Second, an objective measure of deviation: IV Rank or IV Percentile for volatility, distance in standard deviations from the mean for a spread. Third, and most important, an invalidation criterion: what fact would demonstrate the relationship has broken rather than merely stretched. Without that third element, every reversion trade becomes, by default, holding a losing position indefinitely.
Options structures that exploit it
Options are the natural vehicle for trading volatility reversion because they express it directly. With high IV Rank, premium-selling structures — iron condors, short strangles, credit spreads — profit if volatility compresses toward its mean, even if price moves somewhat. With low IV Rank, buying structures — calendars, debit spreads, long straddles — profit if it expands. Calendar spreads are especially pure for this thesis because they isolate volatility from direction reasonably well. In all cases the survival rule is the same: use defined risk, because reversion is a statistical tendency in aggregate rather than a guarantee in any episode, and the episode that does not revert always arrives.