Options expiration effects

Options expire on a calendar, and the hedging around those dates produces recognisable patterns in the index. None of them are large enough to trade on alone. All of them are worth knowing, because they explain why some days behave oddly and because they interact with everything else in this module.

The calendar

The daily expirations for the S&P 500 and similar products mean there is now an "expiration" every session. That has reduced the distinctiveness of the monthly date but not removed it, because monthly open interest is still far larger.

Effect one: pinning

In the last session or two before a monthly expiration, price in heavily-optioned stocks and indexes shows a tendency to settle near strikes with large open interest. The mechanism is the long-gamma dampening from the previous lesson, concentrated: as expiration approaches, gamma at the nearest strike becomes very large, and dealer hedging around it pushes price back toward the strike whenever it drifts away.

        strike 4,500 (huge open interest)
price   ──╱╲──╱‾╲──╱╲──╱╲╱‾╲──╱╲──  ← oscillates around the strike into the close
        Thursday pm            Friday close

Pinning is a statistical tendency, not a rule. Studies find it measurable but modest, stronger in individual stocks with concentrated open interest than in the index, and easily overwhelmed by news. Practical use: on the day before a monthly expiration, a breakout away from a heavily-optioned strike has a worse follow-through rate than usual, and a fade back toward the strike has a better one. Size accordingly, or simply avoid breakout entries on those afternoons.

Effect two: the unpinning after expiration

When the monthly options expire, the gamma that was pinning price disappears. The Monday after OPEX often sees larger moves than the days before, because the dampening is gone and the dealers' hedges are being unwound or rolled. Some of the sharpest one-day declines and rallies of recent years have landed in the week after a monthly expiration.

This is not a directional edge. It is a volatility edge: expect more range in the week after OPEX than in the week before, and set stops and targets with that in mind.

Key idea: Into monthly expiration, large open interest dampens moves and pulls price toward big strikes. After expiration the dampening disappears and range tends to expand. It is a volatility pattern, not a direction pattern.

Effect three: vanna and charm flows

Two Greek terms appear in expiration commentary. They describe why hedging flows change even when price does not.

Charm is the change in an option's delta as time passes. Out-of-the-money puts lose delta as expiration approaches, so dealers who are short those puts (and hedged by being short the underlying) need progressively less hedge and buy back underlying over the days into expiration. In a calm market this produces a mild, persistent bid into OPEX, which some traders call the "OPEX drift".

Vanna is the change in delta as implied volatility changes. When the VIX falls, out-of-the-money puts lose delta and the same dealers buy back hedges. Falling volatility therefore produces buying, which supports price, which lowers volatility further: the mechanism behind the grinding, low-vol rallies that seem to defy every bearish argument.

Both flows reverse in stress. Rising volatility raises put deltas, dealers sell more underlying, and the decline feeds on itself. That is the short-gamma amplification from the previous lesson, seen through the lens of volatility rather than price.

As with gamma, these are estimated effects built on assumptions about who is on which side. They are consistent with a lot of observed behaviour and they are not laws.

Effect four: VIX expiration and settlement

VIX derivatives settle on a Wednesday morning against a special opening quotation computed from S&P options. Positions rolling into that settlement can produce odd moves in the VIX (and briefly in the index) on the Tuesday afternoon and Wednesday open. The practical note is simply: do not read a VIX spike or drop on VIX settlement morning as a sentiment signal. Check the calendar first.

Effect five: end-of-month rebalancing

Not an options event, but it lands in the same conversation. Pension funds and balanced portfolios rebalance toward target weights at month and quarter ends. After a strong month for stocks they sell stocks and buy bonds; after a weak one, the reverse. The flows are large and predictable in direction, and the last two sessions of a quarter often show the index moving against the month's trend. Again: volatility and flow context, not a trade on its own.

Putting it on the calendar

A useful habit is to mark on your trading calendar:

• Third Fridays, with quad witching flagged.
• VIX settlement Wednesdays.
• Last two sessions of each month and quarter.

Then, for each, write one line the next morning about what the tape did. Over a year you will have your own record of how much these effects show up in the products you trade, which is worth more than any general claim.

Try it: For the last twelve monthly expirations, record the index's range on the Thursday and Friday of expiration week and on the Monday and Tuesday after. Compute the average range for each pair. Then compare to the average range of all other sessions in the same period. You are testing whether the "pin then expand" pattern exists in your data and how large it is.

Recap

• Monthly expiration is the third Friday; quad witching adds futures in March, June, September and December; VIX settles on Wednesdays 30 days before the next monthly OPEX.
• Pinning near big strikes into expiration is real but modest; breakouts away from big strikes on expiration eve have weaker follow-through.
• After expiration the dampening disappears and range tends to expand; it is a volatility pattern, not a directional one.
• Charm and vanna describe hedging flows that drift with time and volatility; calm markets get a grinding bid, stressed markets get an amplified offer.
• Mark the dates, log what happened, and build your own record rather than trusting general claims.