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SVQ · Volatility surface

Put skew and crash risk

Also called: downside skew, crash premium, put wing

Put skew is the extra implied volatility on downside strikes, a market price for crash risk that rises when demand for protection grows.

Definition

Put skew is the amount by which out-of-the-money puts trade at higher implied volatility than at-the-money options for the same expiry. It is the most visible part of the equity volatility smile.

Because those puts pay off in sharp sell-offs, the size of the skew is often read as the market's price of crash risk.

Intuition

Two forces create put skew. First, equity volatility tends to rise when prices fall (the leverage effect), so downside scenarios are also high-volatility scenarios. Second, institutions hedge long portfolios with puts, and the dealers who sell that protection charge for gap risk they cannot hedge continuously.

Since the late 1980s equity index markets have shown persistent downside skew. It changes in steepness rather than disappearing, which makes its level relative to history more informative than its presence.

How it's calculated

Typical measures for one expiry:

  • Put skew25 = σ(25Δ put) − σ(ATM): how much richer the 25-delta put is than the at-the-money option.
  • RR25 = σ(25Δ call) − σ(25Δ put): a risk reversal; more negative means steeper downside skew.
  • Ratio forms such as σ(25Δ put) / σ(ATM), which scale out the overall level of volatility.

Model-free alternatives use the whole put wing, for example the risk-neutral skewness implied by out-of-the-money option prices, or the probability mass in the left tail of the risk-neutral density.

Example: if the 25-delta put trades at 24% implied volatility and the at-the-money option at 18%, the put skew is 6 points, a ratio of about 1.33. A move to 26% against an unchanged 18% steepens it to 8 points.

How to read it

A steepening put skew, especially when at-the-money volatility is calm, says hedgers are paying more for the left tail. A flattening skew can mean either complacency or that a sell-off has already happened and protection has been monetized.

Compare skew against the same tenor's history and against realized downside moves. Skew is a price, and prices can be rich or cheap relative to what later happens.

Common mistakes

  • Treating steep skew as a signal that a crash is likely. It is the cost of insurance, and insurance is often expensive precisely when nothing happens.
  • Ignoring the level of volatility. A fixed volatility-point skew means different things at low and high overall volatility.
  • Using far wings with no two-sided quotes; the measure then reflects the fitting model more than the market.
  • Comparing skew across assets with different typical shapes, such as an index and a single stock.

In Saqrid

SVQ's Crash-O-Meter gives a per-symbol reading of put-skew crash fear, shown next to the volatility risk premium.

The Risk Lab adds tail metrics, wing arbitrage checks and implied-moment time series.

Open SVQ.

  • Volatility smile and skewThe volatility smile and skew describe how implied volatility varies with strike for a single expiry, revealing how the market prices tail moves.
  • Risk-neutral density (Breeden–Litzenberger)The risk-neutral density is the probability distribution of the future underlying price implied by option prices, recovered from their curvature in strike.
  • Volatility risk premium (VRP)The volatility risk premium is the typical gap between implied volatility and the volatility that is later realized, a price for bearing volatility risk.
  • Tail dependenceTail dependence measures the chance that two assets suffer extreme moves together, which ordinary correlation can understate badly.

Go deeper

Saqrid Institute lesson: The Crash-O-Meter: Pricing Tail Fear. Lesson text is open to read without an account.

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