The Volatility Smile and SPX Skew
If Black-Scholes were a perfect description of pricing, every strike at an expiration would share one IV. They don't. The smile is the symmetric version; the SPX skew is the asymmetric version where OTM puts are systematically pricier than OTM calls. What the shape looks like, why it exists, and how it shifts the math of every SPX strategy.
If Black-Scholes were a perfect description of how options are priced, every strike at the same expiration would share one IV. They don't. The pattern of IV across strikes is the volatility smile (symmetric, U-shape) or skew (asymmetric). SPX shows a strong asymmetric skew: OTM puts are systematically pricier (higher IV) than OTM calls of comparable moneyness.
This article covers what the shape looks like, why it exists, how traders measure it, and how it changes the economics of every SPX options strategy.
The flat-IV world that doesn't exist
Black-Scholes assumes one volatility for the option's life. If markets agreed with the model exactly, every strike at the same expiration would share one IV. A chain might look like:
| Strike | IV (hypothetical flat) |
|---|---|
| 4900 | 12.0% |
| 4950 | 12.0% |
| 5000 | 12.0% |
| 5050 | 12.0% |
| 5100 | 12.0% |
The chain you actually see looks nothing like that. IV varies strike-by-strike, in a shape that's distinct enough to have its own name.
What the smile looks like
A U-shaped or "smile"-shaped pattern of IV across strikes, where OTM puts and OTM calls both have higher IV than ATM strikes. Most common in currency and commodity options. The curve sags in the middle and rises on both sides.
The "smile" name comes from the U-shape: IV at the wings is higher than IV at ATM. Currency and some commodity options show this fairly symmetric smile. The market is pricing in the possibility of significant moves in either direction, with extra premium on far-from-ATM strikes relative to what flat-vol BSM would predict.
The smile shape was the first pattern noticed in deviation from BSM, after the 1987 crash made it clear that markets don't actually price options as if returns are perfectly lognormal.
The SPX skew is asymmetric
For SPX (and most equity indices), the shape isn't symmetric. OTM puts have IV substantially above ATM. OTM calls have IV at or slightly below ATM. The "smile" is really a "smirk" — heavily tilted toward the put side.
An asymmetric IV-by-strike pattern. In equity indices like SPX, IV rises sharply for OTM puts and stays flat or declines slightly for OTM calls. The curve looks like a downward-sloping line from far-OTM put to far-OTM call, with a knee near ATM.
The chart below shows a parameterized SPX-style skew. Drag the put-side slider higher to see what a steep crash-pricing regime looks like; drag the call-side slider to see how mild the call-side skew typically is. The 5%-OTM stat boxes show the IV at strikes 5% away on each side.
- 5% OTM put IV
- 17.0%
- ATM IV
- 12.0%
- 5% OTM call IV
- 12.5%
Notice the asymmetry. With moderate skew settings, the 5%-OTM put has substantially higher IV than the 5%-OTM call. That isn't a quoting quirk — it's the market pricing puts more aggressively than calls. The same dollar distance from spot is not the same priced-risk distance.
Why the skew exists
Three contributing reasons traders cite:
Crash risk pricing. Equity returns are empirically negatively skewed: markets fall faster than they rise. The historic sample includes single-day declines (October 1987, August 2015, March 2020) that are far larger than any comparable single-day rally. Options markets price this asymmetry by making downside protection more expensive than upside speculation.
Hedging demand. Pension funds, insurance companies, and anyone with substantial long equity exposure buys OTM put options as crash insurance. That persistent buying pressure on OTM puts supports their price. Meanwhile, OTM call demand is less structural — covered calls and call buying don't generate the same systematic pressure.
Behavioral asymmetry. Loss aversion in market participants tilts attention toward downside risk. Combined with the structural and historical reasons above, the result is that OTM puts trade at substantially higher implied volatility than equidistant OTM calls.
How traders measure skew
Three common measures:
25-delta risk reversal = 25Δ call IV − 25Δ put IV. For SPX this is typically negative (calls cheaper than puts in IV terms). A more-negative number means steeper skew. Quoted frequently in volatility research.
The IV difference between a 25-delta call and a 25-delta put at the same expiration. Captures the asymmetry of the skew in one number. Negative for SPX (puts richer than calls); useful for tracking how the skew shape changes over time.
Skew slope = change in IV per percentage move in strike. Sometimes quoted as "100 IV points per 1% strike move on the put side." Steeper slope = more aggressive crash pricing.
Put skew = relative steepness on the put side specifically — the IV difference between, say, the 10-delta put and ATM. Useful when the action is on the downside.
You'll see these measures in volatility research, dealer commentary, and analytics platforms. Most retail trading interfaces just show the IV column on the chain and let the trader read the shape directly.
What the skew means for strategies
The skew is built into the economics of every SPX strategy:
Credit put spreads on SPX pay more than the symmetric math would suggest. The short put has elevated IV; you're collecting that elevated premium. The same trade on the call side pays less for equivalent strike distance from spot. This is why many 0DTE retail strategies focus on put-side credit collection — the skew makes the math more favorable.
Iron condors are asymmetric in their credit collection. Equal strike distances from spot don't produce equal credits — more credit comes from the put wing than the call wing. Strategy designs can adjust by either widening the put-side wings or matching by delta rather than by points.
Long OTM put protection is more expensive than long OTM call protection. If you're hedging a long SPX position with puts, you're paying the skew premium. The trade-off: that's exactly what's compensating you for the tail-risk insurance.
ATM straddles vs strangles. A strangle that uses equidistant OTM strikes is asymmetric in implied vol because of the skew. Same dollar distance from spot, different IVs.
Skew on 0DTE
The skew shape persists on 0DTE — OTM puts on SPX 0DTE are still pricier than equivalent-distance OTM calls. The exact shape can be more or less pronounced depending on market conditions and the specific events priced into the day's expiration.
The intraday behavior of the skew is its own topic — the skew can steepen during selloffs and flatten during rallies, sometimes shifting noticeably during a single trading session. That dynamic is what 0DTE traders running put-side credit spreads are quietly betting on: that the skew premium they collected at entry will hold or compress favorably over the session.
Key takeaways
- A flat-IV chain doesn't exist in real markets. IV varies strike-by-strike in a recognizable pattern.
- The volatility smile is a U-shaped pattern (common in FX and commodities); the SPX skew is the asymmetric version where OTM puts are substantially pricier than OTM calls.
- Three contributing reasons for the SPX skew: crash-risk pricing (returns are negatively skewed), persistent hedging demand for OTM puts, and behavioral asymmetry around downside risk.
- Traders measure skew with risk reversals (25Δ call IV − 25Δ put IV), skew slope, or put-skew steepness. SPX risk reversals are typically negative.
- The skew is built into every SPX strategy. Put-side credit spreads pay more than the symmetric math would suggest; iron condors are asymmetric in their credit collection; long downside protection costs more than equivalent upside speculation.