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Volatility Skew: Why Implied Volatility Differs by Strike

For educational purposes only; not investment advice.

Direct answer

Section titled “Direct answer”

Volatility skew means options on the same underlying and expiration trade at different implied volatilities across strikes or Deltas. In U.S. equity-index options, downside OTM Puts commonly have higher IV than ATM or comparable upside Calls. That relative premium can reflect crash insurance demand, negative spot-volatility dependence, jump and correlation risk, dealer inventory, capital constraints, and liquidity.

Skew is a relative price shape, not a direct probability forecast. A rich downside Put does not prove that the market expects an immediate decline, nor does it make selling that Put automatically attractive. Its premium can be compensation for losses that arrive when markets, correlation, funding, and execution all deteriorate.

How to measure skew consistently

Section titled “How to measure skew consistently”

Fix the underlying, timestamp, and expiration first. Then choose and disclose a coordinate:

  • Delta skew: compare matched-absolute-Delta OTM Put and Call IV, such as 25Δ Put IV − 25Δ Call IV.
  • ATM-to-wing skew: compare a tail option’s IV with a documented ATM definition.
  • Moneyness slope: compare IV against K/F, log-forward moneyness ln(K/F), or another fixed range.
  • Risk reversal quote: often Call IV minus Put IV, but some vendors reverse the sign.

These are not interchangeable. Delta itself depends on IV, time, rates, dividends, forward, and convention. A “25 Delta” can mean spot or forward Delta and may use premium adjustment. Fixed strikes also change economic moneyness after spot moves. Record the exact definition before comparing days, assets, or vendors.

Do not compare raw Put and Call premiums as skew. Intrinsic value, strike, forward, rates, dividends, and time affect dollar prices. IV translates option prices through a model onto a more comparable scale, but remains model- and input-dependent.

Skew is one slice of a surface. It differs by expiry and can contain event-specific kinks. It can steepen, flatten, rotate, or move with the overall IV level. A single slope cannot describe wing curvature or term structure.

Reading a downside skew

Section titled “Reading a downside skew”

For one expiry, suppose synchronized executable quotes imply:

Surface point IV
10Δ Put 40%
25Δ Put 32%
ATM 26%
25Δ Call 24%
10Δ Call 25%

Under a Put-minus-Call convention:

25Δ skew = 32% − 24% = +8 volatility points

Under a Call-minus-Put risk-reversal convention, the same market is 24% − 32% = −8 points. Nothing economic changed; only the sign convention did. The 10Δ Put − ATM difference is 40% − 26% = 14 points, showing that the far downside wing is richer than a single 25Δ slope reveals.

If a trader sells the 25Δ Put because its IV is 8 points above the Call, the trade is not an 8-point guaranteed gain. A selloff can move the Put toward ATM, raise the entire IV level, steepen skew, widen Bid/Ask, and increase margin simultaneously. Conversely, buying the rich Put pays for protection and can still lose if the feared move does not occur before time value decays.

Analysis and risk controls

Section titled “Analysis and risk controls”
  • Use synchronized, executable Bid/Ask and reject stale, crossed, zero-bid, or anomalously wide wing quotes.
  • Estimate forward, dividends, rates, and time consistently before calculating moneyness and IV.
  • Compare the same expiry and coordinate; label Delta definition, ATM rule, sign, and interpolation.
  • Plot more than two points. A risk reversal measures one relative difference, not full smile curvature.
  • Inspect skew by expiry; earnings, macro events, or index rebalancing can create local term and strike features.
  • Stress spot down/up jointly with IV level, skew rotation, term structure, elapsed time, and spread widening.
  • Revalue each leg. Net Vega near zero can leave large skew, Vanna, Gamma, and basis exposures.
  • Size short-wing positions from gap and liquidity stress, not premium received or historical win rate.
  • Include early assignment, expiration, margin, borrow, adjusted deliverables, and the account’s ability to carry resulting shares.
  • Attribute P&L to spot, time, IV level, skew/curvature, and execution so a directional loss is not mislabeled as skew.

Common misconceptions

Section titled “Common misconceptions”
  • “Put options are always more expensive than Calls.” Dollar prices are not skew; relative IV depends on expiry and coordinate.
  • “Steep downside skew predicts a crash.” It contains risk premiums, demand, inventory, constraints, and beliefs—not one physical probability.
  • “High Put IV means selling is favorable.” Rich compensation can accompany severe negative-Gamma, Vega, gap, and margin risk.
  • “Skew is one universal number.” Sign, Delta, ATM, strike range, expiry, and fit differ.
  • “25Δ skew describes the whole surface.” It misses farther wings, curvature, and term structure.
  • “Fixed-strike comparisons are stable through time.” Spot and forward moves change moneyness.
  • “Net Vega zero removes skew risk.” Different strikes can reprice in opposite relative directions.
  • “A smooth fitted skew is tradable.” Midpoints and interpolated wings may lie outside executable markets.
Section titled “Related topics”

Authoritative sources

Section titled “Authoritative sources”