What is volatility skew in options?

Where the slope comes from

A simple options model assumes a single volatility for every strike. If markets behaved that way, the implied volatility of every option on the same expiry would be identical. They are not, and the differences are systematic.

In equities, the persistent premium on downside puts reflects two realities. First, large declines tend to be sudden and accompanied by rising volatility, while rallies are usually slower and calmer. A put that pays off in a crash is paying off in a high-volatility world, so it is worth more than a model assuming constant volatility would say. Second, there is steady demand for portfolio protection: institutions buy index puts as insurance, and that demand pushes their prices up.

The pattern became a lasting feature of equity index options after the October 1987 crash. Before then, the skew in index options was much flatter. Since then, the market has continued to charge a premium for downside protection.

How skew is measured

There is no single skew number, but a few measures are standard. One compares the implied volatility of a put at a given delta, often 25 delta, with that of a call at the same delta; the difference is called a risk reversal. Another compares a 25-delta put with the at-the-money option. A third measures the slope of implied volatility against strike or moneyness.

Using deltas rather than fixed strikes keeps the comparison consistent as the underlying moves and as volatility changes. A put five percent below the market can be a 25-delta option on one day and a 15-delta option on another, depending on how much volatility and time remain.

Some index providers also publish skew-based measures derived from the prices of far out-of-the-money index options, which estimate how much the market is paying for tail protection specifically.

Skew outside equity indices

Not every market slopes the same way. Single stocks usually have a put skew, but it is often flatter than the index skew, and stocks with heavy speculative call buying can show the opposite: out-of-the-money calls priced at higher implied volatility than puts. That call skew appeared prominently in some heavily traded growth and meme stocks during periods of intense call demand.

Commodities can skew either way. Crude oil and natural gas sometimes show call skew, because supply shocks can push prices sharply higher. Currency options often show a more symmetric smile, with both tails priced above the at-the-money level, leaning toward whichever currency the market fears will weaken.

The shape of the skew is therefore a summary of where a market thinks its fastest, most disorderly moves are likely to come from.

Why skew moves and why it matters

Skew changes with conditions. It often steepens when demand for protection rises, as investors pay up for puts, and it can flatten in a sharp selloff once at-the-money volatility catches up with the put wing. It can also flatten during strong rallies when call demand rises.

For option traders, skew determines the relative price of different structures. A put spread that buys a nearer put and sells a further one benefits from the further put's higher implied volatility. A risk reversal that sells puts to buy calls is often financed by the skew. Ignoring skew means misjudging what each leg of a trade costs.

Skew also feeds dealer positioning. Because downside puts carry higher implied volatility, their gamma and vanna profiles differ from calls at the same distance, and models that use a single volatility for every strike produce a different gamma map from those that use the actual skew.

Reading a change in skew

A change in skew is often more informative than its level. If downside put volatility rises while at-the-money volatility stays flat, the market is paying more specifically for crash protection, which suggests concern about tail events rather than about ordinary movement. If at-the-money volatility rises and the skew flattens, the market is pricing more movement overall without a particular fear of the left tail.

Neither change predicts a move. Skew can stay steep through long rallies, because institutions keep buying protection on rising portfolios. What skew describes is the price of insurance and how that price is changing, which is useful context for anyone buying or selling options in either wing.