Volatility Skew: Why OTM Puts Cost More Than Calls

Key Takeaways

• Volatility skew is the asymmetric IV pattern across strikes; equity indices show persistent negative skew (OTM puts > OTM calls) since the 1987 crash.
• The 25-delta risk reversal for SPX typically runs -4 to -10 vol points, measuring the spread between 25-delta put IV and 25-delta call IV.
• A common mistake: treating skew as a pricing error to fade, it reflects real jump risk and structural hedging demand, and has persisted for decades without reverting.
• Single-stock skew can be positive near catalysts like FDA decisions or takeover bids, where upside surprise risk exceeds downside, unlike index skew.

What It Is

If Black-Scholes held exactly, every option on the same underlying and expiration would trade at the same implied volatility. It does not. Plot IV against strike and you get a curve. For equity indices, that curve slopes downward, with low-strike (OTM put) IVs well above high-strike (OTM call) IVs. The asymmetric pattern is called skew. A symmetric U-shape, more common in currencies, is called a smile.

The academic literature distinguishes two causal explanations: (1) the underlying's return distribution is non-Gaussian, so Black-Scholes systematically misprices options in the tails, and (2) supply and demand from hedgers creates persistent price pressure that shows up as IV patterns.

The Intuition

Prior to the 1987 crash, equity index option surfaces were roughly flat, consistent with Black-Scholes. The crash changed that permanently. One single day of a roughly 22 percent decline repriced the market's estimate of left-tail probability. Post-crash, OTM puts became structurally more expensive than OTM calls, and the negative skew has persisted for decades.

Two forces keep it there. First, the data now shows that jumps and fat tails are real, so any sensible model needs to price a distribution with a heavier left tail. Second, long-only asset managers systematically buy index puts for portfolio protection, and the demand is inelastic: a pension fund hedging tail risk does not stop buying puts because IV is high. That persistent order flow props up the left wing of the surface.

How It Works

A common metric is the 25-delta risk reversal, which is the IV difference between a 25-delta call and a 25-delta put of the same maturity:

RR_25d = IV(25-delta call) - IV(25-delta put)

For SPX, this is typically negative, often in the range of -4 to -10 vol points. A more negative number means a steeper skew. The risk reversal is one of the cleanest single-number summaries of skew regime.

Skew patterns by asset class:

• Equity indices (SPX, NDX, RUT): persistent negative skew. OTM puts > OTM calls in IV terms. Crash-insurance demand plus fat-tailed returns.
• Single stocks: mixed. Most large caps show negative skew like the index. Small caps or takeover targets can show positive skew when upside surprise risk dominates (a biotech awaiting FDA, a target mid-bidding-war).
• FX: often symmetric smile. Both tails get bid up because currencies can move either direction on macro surprises. Some pairs show skew tied to risk-on/risk-off regimes (USDJPY tends to show a smile biased toward yen-strength strikes).
• Commodities: often positive skew in grains and energy, driven by supply-shock upside risk. A drought, OPEC cut, or hurricane spikes the upside.
• Rates: typically flatter and more symmetric, though shifts with policy regime.

Worked Example

Consider one-month SPX options with SPX at 5,000 and ATM IV at 14. Observed IVs across strikes might look like:

Strike 4,500 (90% K)  : IV 20
Strike 4,750 (95% K)  : IV 17
Strike 5,000 (100% K) : IV 14
Strike 5,250 (105% K) : IV 12
Strike 5,500 (110% K) : IV 11

The downward slope is the negative skew. The 25-delta call might sit near strike 5,150 at IV 13 and the 25-delta put near 4,850 at IV 16. The risk reversal is then roughly -3 vol points, modest by historical standards.

Interpretation: the market prices larger downside moves as more likely per unit of move size than upside moves. A 10 percent down-move has higher probability than a 10 percent up-move. That is partly a statement about actual return distributions (negatively skewed) and partly about hedging demand (puts bid by portfolio protectors).

Common Mistakes

1. Treating skew as a pricing error. The skew is not an arbitrage. It reflects real jump risk plus real hedging demand. Strategies that sell OTM puts "because their IV is too high" collect premium every day the market stays calm and then lose years of premium in a single crash episode.

2. Interpreting a single risk reversal in isolation. Skew shifts with regime. A -3 RR in a low-vol bull market is ordinary. A -3 RR in a panic where ATM IV is 40 is unusually flat and might indicate forced selling or put unwinding. Context matters.

3. Confusing single-stock skew with index skew. Single names often have positive skew around catalysts (earnings, trials, buyout speculation). Treating a single-stock surface like an index surface can lead to mispricing on the wrong side of the smile.

4. Forgetting that skew changes with maturity. Short-dated skew steepens in stressed markets because near-term tail risk is front and center. Longer-dated skew is flatter because over multi-year horizons the left-tail premium spreads out. Compare apples to apples by maturity.

5. Assuming skew mean-reverts quickly. Skew can trend for years. The post-1987 skew has never reverted to its pre-crash flat shape. Betting on skew normalization without a structural reason (regulatory change, shift in hedging demand) has historically been a slow-bleed trade.

Frequently Asked Questions

Q: What is volatility skew in simple terms? Volatility skew is the pattern where different option strikes carry different implied volatilities even on the same underlying and expiration. For equity indexes, out-of-the-money puts are persistently more expensive in IV terms than out-of-the-money calls.

Q: How does volatility skew affect investment decisions? When selling OTM puts, you collect a premium inflated by skew, but you are also underwriting the crash risk that the market is pricing. Check the 25-delta risk reversal before selling: a -8 RR means puts are unusually rich on a historical basis.

Q: What is a real-world example of volatility skew? SPX at 5000 with ATM IV at 14. The 4500-strike (90% K) put trades at IV 20 and the 5500-strike (110% K) call trades at IV 11. That 9-point gap, with puts much richer than calls, is the equity put skew in one number.

Q: How can investors use skew information practically? Compare the current 25-delta risk reversal to its 1-year range. An unusually steep skew means crash insurance is expensive, either the market sees near-term risk or there is excess demand for protection. An unusually flat skew might signal complacency.

Q: How is volatility skew different from the term structure of volatility? Skew describes how IV varies across strikes at one expiration. Term structure describes how IV varies across expiration dates at one strike. The full volatility surface combines both dimensions.

Sources

1. Benzoni, L., Collin-Dufresne, P., Goldstein, R. "Explaining Asset Pricing Puzzles Associated with the 1987 Market Crash." Journal of Financial Economics. https://www.sciencedirect.com/science/article/abs/pii/S0304405X11000328
2. Schwert, G.W. "Stock Volatility and the Crash of 87." NBER Working Paper. https://www.nber.org/system/files/working_papers/w2954/w2954.pdf
3. Doran, J.S. (2007). "Is there information in the volatility skew?" Journal of Futures Markets. https://onlinelibrary.wiley.com/doi/abs/10.1002/fut.20279
4. Natenberg, S. Option Volatility and Pricing: Advanced Trading Strategies and Techniques. McGraw-Hill. https://archive.org/details/optionvolatility00shel

Disclaimer

This article is educational content only and is not financial advice. Nothing here is a recommendation to buy, sell, or hold any security. Consult a licensed advisor before making investment decisions.