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Summary

0DTE (zero-days-to-expiration) options — contracts expiring the same day they trade most actively — grew from a curiosity to roughly half of all SPX option volume by the mid-2020s, with SPY showing a similar pattern.[^cboe] All four evidence bundles predate this shift: they treat 30–45 DTE as the natural holding window for premium sellers and discuss gamma as a slow, manageable risk. On expiration day, gamma dynamics dominate and the assumptions underlying TOMIC-style strategies must be re-examined.

Growth of the 0DTE Complex

  • Cboe listed daily-expiring SPX options in 2016, weekly expirations on Monday/Tuesday/Wednesday followed in 2022; SPY added daily expirations in 2022.[^cboe]
  • By 2023–2025, 0DTE contracts routinely accounted for approximately 40–55% of SPX option volume (treat the exact share as approximate; it varies by market conditions).[^cboe]
  • Drivers: retail access to zero-commission trading, intraday index/ETF leverage demand, market-maker willingness to quote tight markets, and the appeal of defined-expiry premium selling with very high annualized theta.
Dimension Book-era baseline (2012) 0DTE era
Typical short-premium DTE 30–45 days [^tomic-greeks] 0–1 days
Gamma over holding life Slow bleed, days of buffer ATM gamma explodes within hours
Theta/day capture Small fraction of credit Often 30–60%+ of the option's remaining value
Adjustment window Days Minutes; frequently no practical adjustment

Expiration Mechanics

  • SPX options are cash-settled in European style: no early assignment, and the AM-settled classic SPX options settle to the opening print (SET) of expiration morning, while the daily/weekly PM-settled series settle to the 4:00 pm ET closing print. SPY options are American-style and physically settled into shares — different assignment dynamics.
  • Near the close, trading in the final minutes effectively determines the settlement price for PM-settled series, which is why end-of-day SPX flows cluster so heavily.

Dealer Positioning and the Rebalancing Loop

  • Dealers who are short 0DTE puts (the most common retail-facing flow is retail selling premium or buying short-dated directional options) hedge dynamically; near expiry, the rate of change of their delta hedge — gamma — becomes very large and changes abruptly with small underlying moves.
  • When dealer gamma is heavily negative into expiration, hedging is pro-cyclical: dealer selling into a down-move and buying into an up-move can amplify intraday trends. When dealer gamma is positive, hedging dampens moves. Conditionally, when dealer gamma is heavily negative into expiration, hedging can amplify intraday trends — but the average effect measured in recent research is the opposite: market-maker intermediation in 0DTE attenuates volatility, and MM net gamma is on average positive and negatively related to subsequent intraday vol.[^ssrn-0dte] Aggregate dealer-gamma (GEX-style) estimates are unvalidated practitioner hypotheses — an 8-year SPX backtest shows GEX largely tracks VIX/ATM IV — and must not gate strategies without out-of-sample validation.[^gex-backtest]
  • Because expiries are daily, this positioning resets and re-concentrates every afternoon, creating recurring intraday flow patterns around large open-interest strikes.

Pin Risk

  • Pin risk: the underlying settling very near a large strike, leaving short options with near-zero delta ambiguity at expiry — the seller cannot know whether an SPY position will be assigned. Classic gamma literature covers this qualitatively,[^natenberg-hedging] but 0DTE concentrates enormous open interest at a handful of strikes every single day, making pins a daily event rather than a rare monthly one.
  • For SPX (cash-settled), pin risk costs money through final-print uncertainty; for SPY (physical settlement), it adds overnight share-assignment risk that a 30–45 DTE seller would manage by rolling long before expiry.
  • Mitigation used by practitioners: close short strikes with meaningful time value remaining before the final hour; avoid strikes where aggregated dealer gamma suggests a pin is likely; treat "hold to expire for the last pennies" as systematically unattractive once assignment/print uncertainty is priced honestly.

Why the Books' 30–45 DTE Assumptions Understate Gamma

  • TOMIC-style guidance assumes gamma risk is small in the first half of a 30–45 DTE trade and grows gradually into expiry.[^tomic-greeks] [^passarelli-gamma] That is true only for long-dated series: gamma scales roughly with 1/(σ√T): at equal IV, a 1-DTE ATM option has ~√45 ≈ 6–7× the gamma of the same-strike 45-DTE option — and intraday, as remaining life compresses to hours, the multiple grows far larger still.
  • Practical consequence: an 0DTE short straddle can be delta-neutral at the open and several hundred deltas wrong within an hour. The books' adjustment cadence (daily checks, rolling at defined loss thresholds) is far too slow; 0DTE requires intraday monitoring, hard pre-commitment to exit points, and position sizes small enough that a full gap through the short strike is survivable.
  • The theta-for-gamma trade is therefore qualitatively, not just quantitatively, different: annualized returns look enormous precisely because the seller is carrying explosion-level gamma for hours. Any EV comparison must include this tail exposure, per the EV contract.

Links

References

  • [^cboe] Cboe Global Markets, Options Microstructure and 0DTE resources: https://www.cboe.com/us/options/microstructure/ (includes Cboe research notes on SPX/SPY 0DTE volume statistics).
  • [^tomic-greeks] Chen, M. & Sebastian, J., The Option Trader's Hedge Fund, 2012 — chapter on the Greeks and trade management.
  • [^passarelli-gamma] Passarelli, D., Trading Option Greeks, 2nd ed., 2012 — chapter on gamma.
  • [^natenberg-hedging] Natenberg, S., Option Volatility and Pricing, 1994 — hedging and pin risk discussion.
  • SEC (Securities and Exchange Commission), Equity Options Market Structure remarks and staff reports discussing growth of short-dated options, 2023: https://www.sec.gov/news/speech.htm