CBOE Skew Index
The CBOE Skew Index, often called the Black Swan Index, is a measure of the perceived tail risk in the S&P 500 derived from options market pricing, specifically capturing how much extra premium the market historically paid for out-of-the-money put options relative to at-the-money options as an indication of left-tail crash risk concerns.
Standard options pricing models assume that asset returns follow a roughly normal (bell curve) distribution. In practice, observed market returns exhibit negative skewness — large downside moves occur more frequently than a normal distribution would predict, a phenomenon sometimes described as having a fat left tail. The CBOE Skew Index measures the degree to which S&P 500 options market participants are pricing in the risk of such left-tail events at any given moment.
The Skew Index is calculated by the Chicago Board Options Exchange using the differential pricing of out-of-the-money (OTM) puts relative to at-the-money (ATM) options. When investors are concerned about the possibility of a sharp sudden market decline — a black swan event — they bid up the prices of deep OTM put options, which would only be profitable in a severe crash scenario. This relative elevation of OTM put premium increases the implied skewness of the return distribution, pushing the Skew Index higher.
The Skew Index is constructed to read approximately 100 when options market pricing implies a roughly normal return distribution with no significant tail risk premium. Readings above 100 indicate that options markets were pricing in some degree of left-tail risk, while readings significantly above 130 or 140 historically have corresponded with periods of elevated concern about catastrophic downside scenarios. A reading near 100 or below has historically indicated relatively low crash risk premium in options pricing.
Interestingly, the Skew Index does not always move in the same direction as the VIX. The VIX measures at-the-money implied volatility across the S&P 500 options surface, capturing broad market uncertainty. The Skew Index specifically captures the steepness of the options volatility smile — the degree to which deep OTM puts are priced more expensively than ATM options on a relative basis. It is possible for the VIX to be low (indicating low broad volatility expectations) while the Skew Index is high (indicating elevated concern about extreme tail events), and this combination has historically attracted particular attention from risk analysts.
The practical interpretation of the Skew Index is nuanced. High Skew readings can reflect genuine institutional hedging activity — large portfolio managers buying deep OTM puts as tail-risk insurance — rather than necessarily an imminent market event. Some analysts have noted that persistently high Skew readings, while indicating elevated demand for crash protection, have not historically translated directly into near-term market weakness. The Skew Index measures what the options market is collectively willing to pay for black swan protection; it does not measure the objective probability of such events occurring.
For students of derivatives and market risk, the CBOE Skew Index provides a window into how professional market participants are thinking about distributional risk at the extremes — a perspective that standard volatility measures like the VIX do not fully capture.