Zero-Day Options and the Argument About Intraday Volatility

Options expiring on the day they are traded have grown from a curiosity to a substantial share of index options volume. The structural change is real. The market consequence is disputed, and the dispute is worth understanding because both sides have evidence.

The amplification argument runs as follows. An option expiring in hours has enormous gamma near the strike, because its delta swings from near zero to near one over a very small move in the underlying. Dealers hedging a large book of such options must trade the underlying aggressively as price approaches key strikes, and if dealers are net short these options, that hedging is directional in the same direction as the move. The mechanism is real and the mathematics is not in question.

The dampening argument runs the other way. If customers are net sellers of same-day options rather than buyers, dealers are net long gamma, and dealer hedging then leans against moves rather than with them. There is evidence that a substantial share of same-day volume is systematic premium selling, which would place dealers on the long-gamma side and imply a stabilising rather than destabilising effect.

Why the evidence is hard to settle

Both mechanisms are real and they operate simultaneously on different parts of the same market. The net effect on any given day depends on the balance of customer flow, which is not directly observable. Studies have reached different conclusions partly because they cover different periods with different flow compositions, which is a reason to hold the question open rather than to pick the answer that fits an existing view.

What is not in dispute is that the concentration of expiry risk into a single session compresses the window in which positioning resolves. Whatever the sign of the effect, its timing has changed: exposure that previously unwound over weeks now unwinds within hours, and that compression is itself a change in market structure worth accounting for.

What a Practitioner Should Actually Do With This

The useful response to a contested mechanism is not to pick a side but to change how positions are sized around the uncertainty. If the intraday distribution has genuinely fattened at the extremes, then strategies that rely on being able to exit at a particular level during the session carry more execution risk than their backtests suggest, because those backtests were fitted to a period with different microstructure.

The conservative adjustment is to assume worse fills during stressed sessions and to avoid strategies whose viability depends on precise intraday execution. This adjustment is correct under either interpretation of the evidence, which is what makes it a better response than resolving the debate prematurely in whichever direction is more convenient.

The Australian market has almost none of this

Same-day expiry options are an equity index phenomenon and, within that, a United States index phenomenon. The ASX 200 index options market does not offer daily expiries, and volume in the weekly contracts is modest by any measure. Local investors are exposed to the mechanic principally through offshore holdings and through the correlation link between the ASX 200 and the S&P 500 during United States trading hours.

The practical consequence is that intraday flow dynamics in the local market look different from what the imported commentary describes. Australian dealers do not carry large same-day option books, and Australian systematic overwriting programmes concentrate in longer-dated tenors. A framework built on same-day gamma mechanics will therefore misdescribe the local environment even when it correctly describes Wall Street the same day.

What has actually changed in the data

Two features distinguish the current regime from the one that existed before same-day options were listed on every index and every day of the week. Intraday realised volatility has fallen on average, while the tails of the intraday distribution have grown fatter. Those two findings together are consistent with dealers being net long gamma most of the time and net short in specific sessions, which is exactly what a mixed customer flow would produce.

Neither finding by itself resolves the debate, and combining them does not either. A calmer distribution with fatter tails describes a market that is quieter on average and more explosive occasionally, which is a genuine change even if the average sign of the effect remains contested. That is worth accounting for regardless of which camp the underlying mechanism eventually favours.