Volatility Skew

What is volatility skew?

Volatility skew is the cross-section of implied volatility across strikes in one expiration. A simple option model can use one volatility input for every strike, but market prices imply different values. Plotting those values against strike or delta produces a skew curve. Extending the exercise across expirations creates a volatility surface.

Equity and index options commonly display downside skew: out-of-the-money puts carry higher IV than at-the-money options and equidistant calls. Other products can show call skew or a two-sided smile. The shape is an observed pricing relationship, not a rule that every chain must follow.

Traders sometimes use "skew" for differences across expirations. Calling that term structure is more precise. Strike skew compares options horizontally within one maturity; term structure compares similar options vertically across maturities.

What causes volatility skew?

Implied volatility is the model input that reconciles an observed option price. If investors pay relatively more for one strike, its solved IV is higher after controlling for the other pricing inputs. Persistent demand for portfolio protection is one reason downside puts can carry a premium. Sellers also price jump risk, asymmetric returns, hedging costs, and inventory into quotes.

The curve changes through trading. A large put purchase can steepen the downside, a rally with call demand can lift the upside, and an event can raise both wings.

How is volatility skew measured?

Skew can be measured by strike, moneyness, or delta. Fixed-delta comparisons are common because a 25-delta option represents a broadly comparable point on different underlyings and dates. Strike comparisons are intuitive but become stale as spot moves. Forward moneyness can improve comparisons across expirations by accounting for carry.

Common shapes include:

• Flat: implied volatility is similar across the selected strikes.
• Smile: both out-of-the-money calls and puts have higher IV than near-the-money options.
• Downside smirk: put IV rises as strikes fall below spot.
• Upside smirk: call IV rises on higher strikes, which can occur in products with upside jump risk.

How traders use volatility skew

Skew is central to strike selection. Two vertical spreads with similar width can have different economics because the purchased and sold strikes sit at different points on the curve. A collar often buys relatively high-IV downside protection and sells a lower-IV upside call. A risk reversal explicitly trades the difference between matched put and call IV.

Traders compare current skew with its own history to identify unusual relative pricing. The comparison holds tenor and delta constant so that a change in the sample does not masquerade as a change in the market. They also examine how skew responds to spot. A fixed-strike curve and a fixed-delta curve can tell different stories as the underlying moves.

Options flow shows what is moving the curve intraday. Aggressive wing buying can move the relative quote before at-the-money IV changes. A trader who watches only one headline IV may miss that the strategy’s purchased strike became expensive compared with the sold strike.

Risk managers use the full surface to price and hedge portfolios. Vanna and other cross-Greeks capture how delta and vega change when spot and the surface move together. One parallel IV shock cannot represent a steepening or flattening skew.

Put-call parity provides a valuable quality check. European-style calls and puts with the same strike and expiration should be consistent after accounting for the forward, rates, and dividends. A large apparent IV difference between the same-strike call and put can indicate stale quotes or model inputs rather than tradable skew. Surface builders commonly use out-of-the-money puts below the forward and out-of-the-money calls above it to reduce those distortions.

Skew trades also carry direction. A risk reversal that buys a call and sells a put has net delta even when its premiums offset. Comparing IV identifies the relative volatility entry, while delta hedging or a deliberate directional view determines how the underlying exposure is handled.

How to compare skew across matching strikes

State the comparison coordinate and subtraction order. Fixed-strike, fixed-delta, and fixed-moneyness views track different aspects of the surface as spot moves.

Compare executable bid and ask premiums as well as midpoint IV, especially in the wings. Keep the expiration and pricing inputs consistent, then identify the richer side and the relative cost of the selected strategy.

Bring each leg’s IV into the payoff calculation. Skew expresses relative option pricing, including risk premia and supply-demand effects. Reprice the position across the planned spot, time, and surface changes.

Limitations of volatility skew

• Read skew as relative pricing between strikes, and trade it when you have a view on which tail is mispriced. Steep put skew prices demand for downside protection; direction comes from flow and price.
• State the convention: the coordinate (strike, delta, or moneyness), the subtraction order, and the expiration. Two systems can report opposite signs for the same chain.
• Price the wings from executable quotes. Wing quotes can be wide or stale, so midpoint IV can misstate what is actually tradable, and a same-strike call and put IV gap can be a data problem rather than skew.
• Refresh the curve as spot moves. A fixed-strike curve goes stale as spot changes, and flows or events can steepen or flatten it quickly.
• Read the cause from options flow. Skew blends protection demand, jump risk, hedging costs, and inventory, and the flow shows which one is moving the curve.

Bottom line

Volatility skew shows how the options market prices different strikes within one expiration, and it can matter as much as the overall IV level for a multi-leg trade. State the coordinate and sign convention, compare executable premiums in the wings, and put each leg’s own IV into the payoff, with direction coming from flow and price.