Options that expire the same day they are traded now account for a majority of S&P 500 index option volume. They did not exist as a daily product until recently, they are invisible to the VIX, and they have quietly changed the shape of the average trading session.
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Short answer: what a zero-days-to-expiry contract is
Short answer: A 0DTE option is one expiring at the end of the current trading day. It is not a special contract type — it is an ordinary option on its final day. What changed is availability: index options now expire every weekday rather than weekly or monthly, so a same-day contract exists every session. Their defining property is extreme sensitivity to the underlying near expiry, which makes them cheap in dollar terms and enormous in delta terms.
Growth from a Friday product to a daily one
SPX options originally expired monthly. Weeklies were added, then Mondays and Wednesdays, and eventually every weekday. Each addition created another expiry that spends its final hours as a 0DTE instrument.
The result compounds. A market with five weekly expiries has a same-day contract every session, and the volume that once concentrated into Friday afternoons now recurs daily. Same-day contracts routinely make up the majority of SPX option volume — a structural feature of the session rather than an event.
Dealer gamma and the pinning effect
This is the mechanism that makes 0DTE matter for people who never trade options.
When market makers sell options to customers, they hedge by trading the underlying. How they hedge depends on their gamma position. If dealers are long gamma, hedging requires selling into strength and buying into weakness — a stabilising flow that dampens moves and pins price near large strikes. If dealers are short gamma, hedging requires the opposite: buying as price rises and selling as it falls, which amplifies moves.
Gamma rises sharply as expiry approaches, so a 0DTE book generates far larger hedging flows per dollar of premium than a longer-dated one. In the final two hours of a session, hedging demand from same-day contracts can exceed the natural order flow in the underlying. That is why price so often gravitates toward a large strike into the close, and why the last hour behaves differently from the rest of the day.
Why 0DTE volume does not show up in the VIX
The VIX is calculated from SPX options with roughly 23 to 37 days to expiry, interpolated to a constant 30-day horizon. Same-day contracts are outside that window by construction.
This creates a genuine measurement gap. An enormous amount of options activity — arguably the most active part of the market — is excluded from the headline volatility gauge. A session with violent intraday swings driven by same-day hedging flows can end with the VIX barely moved, because the thirty-day expectation the VIX measures did not change. Cboe publishes a separate one-day index for this reason, and it behaves quite differently from the VIX.
The afternoon volatility signature
The intraday pattern has shifted in a way that is visible in realised volatility by time of day. The open has always been volatile as overnight information is absorbed. What is newer is a pronounced increase in the final ninety minutes, when gamma exposure peaks and hedging flows are largest relative to available liquidity.
The direction of that effect depends on dealer positioning, which is why the same structure produces both unusual calm and sudden acceleration into the close on different days. Long dealer gamma pins; short dealer gamma amplifies. Estimates of aggregate dealer positioning are published by several vendors and should be treated as models with meaningful error, not measurements.
What the research actually found about fragility
The concern raised when 0DTE volume grew was that it could amplify a shock into a cascade. Research to date has been more equivocal than either the alarmed or the dismissive framing suggests.
Studies from exchanges and academics have generally found that same-day options have not increased realised volatility at the daily level, and that order flow is more two-sided than assumed — buyers and sellers of same-day contracts are reasonably balanced, which limits net dealer exposure. The counterargument is that the sample is short, contains no genuine crisis with the current market structure in place, and that the relevant risk is conditional rather than average. Both positions are defensible; neither has been tested by the event that would settle it.
Mini glossary
- Gamma. The rate at which an option’s delta changes as the underlying moves. Rises steeply near expiry.
- Delta. Sensitivity of option price to a one-point move in the underlying.
- Pinning. The tendency of price to gravitate toward a large open-interest strike into expiry.
- Charm. The change in delta with the passage of time — a significant hedging driver on expiry day.
What this article does not conclude
Nothing here claims that same-day options make markets more dangerous or that they are harmless. The structure is new enough that the evidence base is thin, and the honest position is that the intraday effects are well documented while the tail risk is genuinely unknown.
Dealer gamma estimates in particular are model outputs published by vendors using proprietary assumptions about who is on which side of each trade. Treat them accordingly.