By the FinLingo Team | Capital markets practitioner, front office experience at a major European investment bank. FinLingo covers 342 lessons from bonds to exotic derivatives. About · Last updated:
The Black-Scholes model assumes constant volatility across all strikes. In reality, implied volatility varies systematically with strike price. When you plot IV against strike for options with the same expiry, the resulting curve is called the volatility smile (if symmetric) or volatility skew (if tilted). This pattern is one of the most important empirical facts in options markets.
For equity index options, the curve typically slopes downward from left to right: OTM puts (low strikes) have higher IV than ATM options, which have higher IV than OTM calls (high strikes). An ATM option might trade at 20% IV, while a 90% strike put trades at 25% and a 110% call at 18%. This downward skew is sometimes called the "smirk." For FX options, the smile is more symmetric, with both deep ITM and deep OTM options having elevated IV.
Three explanations. Crash risk: investors buy OTM puts for portfolio protection, driving up their prices (and IV). After the 1987 crash, the skew steepened dramatically and never reverted. Fat tails: real returns have heavier tails than the lognormal distribution BSM assumes. Large down-moves are more probable than BSM predicts, so puts protecting against those moves trade at a premium. Supply and demand: systematic demand for downside protection from institutional investors creates persistent buying pressure on OTM puts.
The steepness of the skew tells you how much the market fears a crash. A flat smile means the market sees equal risk of moves in both directions. A steep skew means the market is pricing in significantly higher probability of a large decline. After major sell-offs, the skew typically steepens as investors rush to buy protection. In calm markets, it tends to flatten.
The smile means BSM misprices away-from-the-money options. An OTM put priced at "BSM + smile" costs more than BSM alone would suggest. Dealers who use BSM must apply a volatility correction for each strike — the smile is essentially a lookup table of adjustments to the flat-vol assumption. More sophisticated models (local vol, stochastic vol, jump-diffusion) attempt to capture the smile endogenously.
The volatility smile is the pattern where implied volatility varies across strike prices for options with the same expiry. When plotted, IV forms a curve that can be U-shaped (smile, common in FX) or downward-sloping (skew, common in equities). It reveals that the market assigns different volatility expectations to different price levels, contradicting Black-Scholes constant vol assumption.
Three reasons: crash risk (investors buy OTM puts to protect portfolios, pushing up prices), fat tails (large drops are more likely than Black-Scholes predicts, so the market charges more for protection against them), and institutional demand (pension funds and asset managers systematically buy downside protection). After 1987, this skew became a permanent feature of equity options markets.
A steeper skew means the market is pricing in a higher probability of a large decline relative to normal conditions. During calm markets, the skew is moderate. After sell-offs or ahead of uncertain events, the skew steepens as demand for downside protection spikes. Monitoring skew changes is a key part of vol trading and risk management.
FinLingo covers the volatility surface in Level 3 — 7 units on smile, skew, term structure, and surface dynamics. Level 1 is free.
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