The Foundations of Gamma: What Is Gamma?

In options terminology, gamma measures how an option’s delta changes as the underlying price moves. Delta itself is the sensitivity of an option’s price to a $1 move in the underlying. A call option has a positive delta (ranging from 0 to +1), while a put option has a negative delta (ranging from -1 to 0).

The deeper an option moves in the money, the closer its delta approaches ±1; the further out of the money it sits, the closer its delta is to zero.

Gamma, then, is the “second derivative”: it captures how much an option’s delta evolves with each incremental price movement.

For a call option, if the underlying’s price increases, the call’s delta also rises, and gamma indicates the rate of that increase. When an option is near the money and nearing expiration, its delta changes rapidly with small price moves—a scenario that can lead to heavy hedging flows from traders and market makers.

Why Gamma Leads to Real Buying or Selling

When large volumes of options exist on an underlying index like the S&P 500, market makers who sold those options hedge their risk. They aim to stay “delta-neutral,” meaning the net effect of all their positions has minimal directional exposure. This is done through buying or selling shares (or index futures) corresponding to their net option positions.

Consider a market maker who is short a batch of put options. As the underlying price declines, those put options gain value, and their delta grows more negative. To hedge, the market maker must buy more of the underlying to offset their increasingly negative net delta.

Conversely, if the market rises, the put options’ deltas shrink, and the market maker may sell off some of the underlying. The outcome: gamma-driven flows can exacerbate intraday price swings by adding buying pressure on the way up and selling pressure on the way down, or vice versa.

The Importance of Op-Ex

Gamma’s influence often intensifies around options expiration (Op-Ex) dates, especially quarterly ones (March, June, September, December). During these times, large amounts of options open interest converge at key strikes. As prices hover near these strikes, the “pinning” phenomenon can emerge, with heavy hedging flows keeping the underlying near certain levels. After those options expire, the hedging flows abruptly vanish, sometimes causing market moves to speed up or “unpin.”

One reason the expiration cycle matters is that the largest pools of open interest typically center around quarterly expiries. According to various data analyses, these expiries can represent a significant percentage—sometimes as high as 15% to 20% more than standard months—of total open interest. When that open interest rolls off, the market can enter a new regime where previously stabilizing hedging behaviors evaporate, leaving the market more prone to bigger price swings.

High Volatility vs. Low Volatility Environments

Volatility also plays a part in shaping gamma’s effect. In a high-vol environment, near-the-money options can carry significant premiums, but the range of plausible price moves is wider. This “flattens” the gamma curve, dispersing the effect across multiple strikes. Alternatively, in a calmer, low-vol regime, a narrower band of near-the-money strikes can host large portions of gamma, creating a more concentrated potential for sharp intraday moves if the market lingers near those strikes.

Short Gamma vs. Long Gamma

The term “short gamma” implies a position whose delta exposure increases against the holder’s interest when the underlying moves. For example, if you short a call option, you gain negative gamma: the deeper in the money the option becomes, the more you lose per incremental rise in the underlying.

Short gamma positions, when concentrated among many market participants, can speed up volatility because those participants must “buy high, sell low” to remain hedged.

When the market is net “long gamma,” it’s the opposite. Participants effectively “sell high and buy low.” This often suppresses realized volatility and can lead to a dampening effect on day-to-day price changes. Traders who are net long gamma profit from oscillations, because they constantly readjust deltas in their favor.

Why Gamma Matters More Than Ever

Over time, the notional size of options has grown relative to the equity market’s total market capitalization. Data suggests that S&P 500 (SPX) options open interest can total 20-25% of the index’s entire market cap, spread across puts and calls. When that scale is combined with lower liquidity, short gamma pockets can generate outsize impacts on prices. If an adverse move occurs, dealers might be forced to transact in large size at the worst possible moments.

Conclusion

Gamma may seem like a purely technical concept, but in a market where options have become so integral—and where dealer hedging is an ongoing process—its influence can be quite meaningful.

Whether you’re a fundamentals-focused investor or an active trader, keeping track of large pockets of open interest, strike distributions, and volatility shifts can help in anticipating when gamma-driven flows might bolster or undermine market momentum.

During key expirations, these effects can become especially pronounced, guiding shifts in market behavior that may appear disconnected from standard economic or earnings-driven narratives.