Skew Term Structure: How Event Risk Steepens Short-Dated Skew

Key Takeaways

• Skew term structure is the 25-delta risk reversal plotted across expirations; it is typically steep at short maturities (-6 to -8 on SPX 1-week) and flattens toward longer maturities (-2 to -3 at 2 years).
• Ahead of FOMC meetings, SPX 1-week skew can push to -9 or -10; the day after resolution, it snaps back to -5, a 4-5 vol point collapse in one session.
• A common mistake: selling short-dated skew because it "looks rich" right before a Fed meeting, that premium is compensation for event tail risk, not a stale mispricing.
• Long-dated skew barely moves during event resolution; a trade selling the steep front and buying the flat back captures the skew collapse without much change in the hedge leg.

What It Is

Skew is the asymmetry of IV across strikes at a fixed expiration. The term structure of skew asks a different question: at the same moment, how does skew shape change as you walk the expiration ladder?

A common way to plot it: pick a skew metric (often the 25-delta risk reversal, or the slope of IV against log-moneyness at ATM), compute it for each expiration, and plot against days to expiration. On SPX, the result is typically a curve that starts steep at short maturities, flattens monotonically into longer maturities, and levels off past one year.

The Intuition

Two forces drive the shape. First, distributional scaling. Daily returns are leptokurtic and have jump risk. Over one year, the central limit theorem averages much of that tail behavior out, so the distribution is closer to lognormal and skew is thinner. Over one week, jumps and fat tails dominate, and skew is thick.

Second, event risk. Short-dated options span a small number of known events (a Fed meeting, an earnings release). If the next three weeks include a CPI print and an FOMC meeting, short-dated skew prices the binary downside risk of those events. Longer-dated options span many events, so the tail premium spreads out and any single event contributes less to average skew per expiration day.

The result is short-dated skew that reacts to the calendar of upcoming events, and long-dated skew that reflects steady-state distributional assumptions.

How It Works

A standard skew-term-structure readout on SPX at a given moment:

Expiry      25d RR (IV calls - IV puts)
1 week       -6.0
2 weeks      -5.5
1 month      -5.0
3 months     -4.0
6 months     -3.5
1 year       -3.0
2 years      -2.5

More negative = steeper skew. Each reading is the difference between 25-delta call IV and 25-delta put IV at that maturity. The monotonic flattening is typical in calm markets.

Three dynamics reshape the curve:

1. Pre-event steepening at the short end. Ahead of a known event, the short-dated skew steepens further as put demand rises for tail protection spanning the event date. The week-out RR might push to -9 or -10 on a FOMC week.

2. Post-event collapse. After the event clears, the short-dated RR reverts sharply (often the next morning) back toward its non-event level. Short-dated skew "resolves" while long-dated skew barely moves.

3. Level-change compression. In a spot selloff, vol rises across the curve and skew levels re-equalize at higher vol. Short-dated skew rises in absolute terms but flattens relative to vol, because ATM IV rises faster than OTM-put IV.

Mathematically, the short-dated RR follows a simple stylized pattern:

RR(t) ~= RR_baseline(t) + sum over upcoming events of (event_skew_contribution / t)

The 1/t factor is what makes events matter more at short tenors: dividing the event's skew contribution by small t amplifies it.

Worked Example

Consider SPX two weeks before an FOMC meeting and two weeks before earnings season kickoff, with no other events in the immediate window.

Before the events:

1 week RR:  -7.0  (includes FOMC front-end premium)
2 weeks:    -6.5
1 month:    -5.5
3 months:   -4.0
6 months:   -3.5

The day after FOMC (one event resolved), the short end steps down:

1 week RR:  -5.0
2 weeks:    -5.0
1 month:    -5.0
3 months:   -4.0
6 months:   -3.5

The 1-week RR flattened by 2.0 points overnight. The 6-month RR barely moved. A trader who sold the short-dated put skew (short puts, long calls on 1-week tenor) and bought the 6-month put skew as a hedge captured the event-skew collapse.

The same mechanic holds on single stocks around earnings. A 1-week skew on an index member might be -12 into earnings and -5 a day later, a 7-point move. Longer tenors on the same name might show a 0.5-point move.

Common Mistakes

1. Comparing RRs across unequal tenors. A -5 RR on 1-week and -5 RR on 1-year are not the same thing. The 1-week premium is much higher per unit time. Normalize by the square root of time or by ATM vol if you want a dimensionless comparison.

2. Ignoring the event calendar. Short-dated skew reflects specific dates. A backtest that treats short-dated RR as a stationary time series will misread event weeks as outliers rather than as structural.

3. Selling the steep short end without the event. The premium in the front is compensation for event risk. Selling 1-week skew because "it looks rich" right before a Fed meeting is selling tail insurance before the tail event.

4. Assuming the flattening is monotonic. Most of the time it is, but in stressed regimes the shape can kink. A panic followed by expected policy response can make 3-month skew steeper than 1-week and 1-year skew. Inspect the whole curve, not a single slope reading.

5. Using a single metric. 25-delta RR, 90-110 moneyness butterfly, and ATM-vol skew all capture slightly different aspects of skew. A regime change can show in one metric and not another, so track a few and not just one.

Frequently Asked Questions

Q: What is the skew term structure in simple terms? It describes how the asymmetry of implied volatility across strikes, measured by the 25-delta risk reversal, changes from short-dated to long-dated options. Short-dated skew is usually steeper (more expensive puts vs calls) than long-dated skew, and event weeks push short-dated skew even steeper temporarily.

Q: How does the skew term structure affect investment decisions? It tells you whether short-dated put premium is event-driven or structural. Before a known event, the steep front-end skew prices specific tail risk. After the event passes, that skew collapses. Traders who sell the event-steepened front and buy the flatter back can profit from that normalization.

Q: What is a real-world example of skew term structure dynamics? SPX 2 weeks before FOMC: 1-week RR at -7.0, 1-month at -5.5, 3-month at -4.0. Day after FOMC: 1-week RR snaps to -5.0, while 3-month barely moves to -3.9. A trader who sold 1-week put skew and bought 3-month put skew captures a 2-point RR move on the short end.

Q: How can investors use the skew term structure practically? Compare current short-dated RR to its 60-day average. If it is 2 or more points steeper than average without a known event in the window, it may be rich. If a known event is inside the expiration, treat the extra skew as event compensation, not a fade target.

Q: How is the skew term structure different from the vol term structure? The vol term structure tracks ATM implied vol across expirations. The skew term structure tracks the difference between OTM put IV and OTM call IV across expirations. Both are dimensions of the full volatility surface, but they respond differently to events: vol level rises on stress, skew rises more on event-specific tail fears.

Sources

1. Natenberg, S. Option Volatility and Pricing: Advanced Trading Strategies and Techniques. McGraw-Hill. https://archive.org/details/optionvolatility00shel
2. Sinclair, E. Volatility Trading (2nd ed., 2013). Wiley. https://www.wiley.com/en-us/Volatility+Trading%2C+2nd+Edition-p-9781118347133
3. Derman, E. "Regimes of Volatility." Goldman Sachs Quantitative Strategies Research Notes, January 1999. http://pricing.online.fr/docs/regimes.pdf
4. ORATS. "Volatility Around Earnings." https://orats.com/university/volatility-around-earnings

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.