The Core Idea
In a textbook Black–Scholes world, every option on the same underlying and expiration would share a single implied volatility. In real markets they do not. Plot implied volatility against strike price and you get a curve, not a flat line. That curve is the volatility skew (when it slopes) or the volatility smile (when it curves up on both sides). Its shape encodes what the market truly believes about the distribution of future returns — information a single volatility number cannot capture.
- Volatility smile: Both deep out-of-the-money (OTM) puts and calls trade at higher implied volatility than at-the-money (ATM) options, forming a U-shape. This is typical in currency and some commodity markets where large moves can go either way.
- Volatility skew (or smirk): An asymmetric curve. In equity indices, OTM puts carry markedly higher implied volatility than equidistant OTM calls, producing a downward-sloping line from low strikes to high strikes.
Skew is the difference between forward-looking and historical risk measures made visible across strikes. For the broader contrast between the two, see implied versus historical volatility.
Equity-Index Skew: Puts Richer Than Calls
The S&P 500 option surface has a pronounced, persistent skew: downside puts are systematically more expensive (in implied-volatility terms) than upside calls. A stylized snapshot looks like this:
Illustrative SPX 30-day skew (ATM IV = 16)
| Strike (relative to spot) | Option | Implied volatility |
|---|---|---|
| 90% (10% OTM) | Put | ~22 |
| 95% (5% OTM) | Put | ~19 |
| 100% (at-the-money) | — | 16 |
| 105% (5% OTM) | Call | ~14 |
| 110% (10% OTM) | Call | ~13 |
Values are illustrative, not a live quote. The pattern — falling implied volatility as strikes rise — is the defining feature of equity-index skew.
This shape did not always exist. Before 1987, index option-implied volatility across strikes was far flatter, closer to the Black–Scholes ideal. That changed on Black Monday, 19 October 1987, when the Dow fell about 22.6% in a single session. The crash demonstrated that index returns have fat left tails that the lognormal model badly underestimated. Ever since, OTM puts have carried a permanent crash-risk premium, and the negative skew has become a structural fixture of equity-index markets.
How Skew Reflects Crash Fear
The skew is the option market's pricing of asymmetric risk. Equity indices fall faster than they rise: panics are violent and correlated, rallies are gradual. The skew encodes three linked beliefs:
- Downside is fatter-tailed than upside. The probability the market assigns to a large drop exceeds the probability of an equally large gain, so OTM puts are bid up.
- Volatility rises when prices fall. A market decline tends to coincide with a volatility spike, which inflates the value of puts further. This leverage/feedback effect steepens the skew.
- Hedging demand is one-sided. Institutions overwhelmingly buy puts to protect long portfolios; far fewer participants buy upside calls for the same insurance purpose. That structural imbalance is the same force behind the volatility risk premium.
When fear intensifies, the skew steepens — the left side of the curve lifts relative to the right. A steepening skew can therefore be a more granular fear signal than the VIX level alone.
The Cboe SKEW Index
Cboe distills the shape of the left tail into a single number: the SKEW Index. It measures the perceived tail risk of S&P 500 returns over a 30-day horizon, derived from the prices of OTM S&P 500 options. Where the VIX captures the overall level of expected volatility, SKEW captures the asymmetry — how much extra the market is paying for deep downside protection.
| SKEW reading | Interpretation |
|---|---|
| ~100 | Near-normal return distribution; low perceived tail risk |
| ~115–130 | Typical elevated demand for downside protection |
| >130–150 | Heightened concern about an extreme negative move |
Historically the SKEW Index has ranged from roughly 100 to 150. A value near 100 implies the market is pricing returns as approximately normal; higher values flag growing demand for crash insurance. SKEW is best read alongside the VIX and other gauges rather than in isolation, since a high SKEW with a low VIX means very different things than a high SKEW with an already-elevated VIX. Our overview of VIX-related indicators covers SKEW alongside VVIX, VIX9D, and VIX3M.
Reading SKEW carefully
SKEW is not a timing signal on its own. High readings indicate the market is paying up for tail protection, which can persist for long stretches without a crash materializing. Treat it as a sentiment and positioning gauge, not a trade trigger. This is educational information, not investment advice.
How VIX Differs From the Full Surface
The VIX is a single 30-day implied-volatility figure for the S&P 500, computed from a strip of OTM SPX options. By construction it summarizes the whole surface into one near-at-the-money level; it does not, on its own, tell you whether that level comes from a steep skew or a flat one. For the exact methodology, see how the VIX is calculated.
Two markets can post the same VIX while having very different skews. That is precisely why traders watch SKEW and the surface together: the VIX gives the average altitude, the skew gives the slope. Both are needed to understand the cost of any given option structure.
How Skew Affects Pricing and Spread Selection
Because implied volatility varies by strike, skew directly shapes the cost and payoff of multi-leg strategies. A few practical consequences:
- Put spreads. Buying a near-the-money put and selling a lower-strike put means buying lower implied volatility and selling higher implied volatility — the skew works partly in your favor on the short leg, making put debit spreads relatively more attractive than a single long put.
- Risk reversals. Selling an OTM put to fund an OTM call exploits the skew directly: you collect the richer put implied volatility and pay the cheaper call implied volatility. The trade-off is taking on the downside the skew is pricing.
- Calendar spreads. Skew differs by expiration, so calendars are sensitive to changes in the skew's term structure, not just its level.
- VIX options. Skew also appears in the VIX option surface, but it slopes the other way — VIX calls are typically bid relative to puts because traders hedge volatility spikes. See our VIX options guide for that distinction.
Ignoring skew leads to mispriced expectations: a strategy that looks cheap at a flat-volatility assumption may be expensive once the real surface is applied, and vice versa.
Frequently Asked Questions
What is the difference between skew and a smile?
A volatility smile is roughly symmetric: both deep out-of-the-money puts and calls carry higher implied volatility than at-the-money options, common in currencies. A skew (or smirk) is asymmetric: in equity indices, out-of-the-money puts carry markedly higher implied volatility than equidistant calls, producing a downward-sloping curve.
Why are index puts more expensive than calls?
Equity-index returns are negatively skewed and crash downward far faster than they rally. Since the 1987 crash, investors have paid a persistent premium for downside protection, so out-of-the-money puts trade at higher implied volatility than out-of-the-money calls.
What does the Cboe SKEW Index measure?
The Cboe SKEW Index measures the perceived tail risk in S&P 500 returns over a 30-day horizon, derived from out-of-the-money option prices. It typically ranges from about 100 to 150; a reading near 100 implies a near-normal return distribution, while higher readings signal greater demand for crash protection.
How is skew different from the VIX?
The VIX summarizes the overall 30-day implied volatility level of the S&P 500 across a strip of strikes, producing a single number near at-the-money. Skew describes the shape of the implied-volatility curve across strikes — information the single VIX level cannot convey on its own.