Why does negative gamma accelerate selloffs?
Negative gamma accelerates selloffs because dealers who are short gamma must sell the underlying as it falls in order to stay hedged, and their selling pushes the price lower, which requires more selling. On equity indices the effect is often reinforced by rising implied volatility, which enlarges put deltas, and by thinner liquidity as market makers widen their quotes. The result is a market in which declines that start tend to travel further and faster than they would otherwise.
The core loop
Under the common assumption that customers are net long index puts and dealers are net short them, dealers carry short gamma at strikes below the market. A short put has positive delta: it loses value as the market falls. Dealers hedge that by holding a short position in futures or the underlying.
As the market falls toward those strikes, the puts' deltas grow in magnitude. The dealers' exposure becomes more positive, their existing short hedge is no longer large enough, and they sell more. The further the decline, the steeper the increase in delta, and the more they have to sell. That is short gamma: the hedge has to grow in the direction of the move.
The selling does not depend on anyone's view of the market. It is forced by the arithmetic of the hedge, and it arrives precisely when other participants are also selling.
Volatility makes it worse
In equities, implied volatility tends to rise as prices fall. Rising volatility increases the delta of out-of-the-money puts, so dealers short those puts see their exposure grow even before price reaches the strikes. That is the vanna effect, and in a decline it adds to the selling that gamma already requires.
Rising volatility also raises the value of the puts dealers are short, which increases their mark-to-market losses and can prompt them to reduce risk more broadly. And it widens the range of outcomes the market is pricing, which leads market makers to quote smaller sizes and wider spreads.
The combination of gamma and vanna is why a decline into heavy short-gamma territory can feel self-reinforcing: price falls, volatility rises, both enlarge the hedge, and the hedging pushes price lower.
Liquidity thins at the worst time
Market depth, the size available at each price level, tends to fall during stress. Market makers facing higher volatility and larger inventory risk show less size. That means each unit of forced selling moves the price further than it would in calm conditions.
Other mechanical sellers can join at the same time. Strategies that target a fixed level of volatility reduce exposure when realized volatility rises. Leveraged products rebalance near the close in the direction of the day's move. None of these are options hedges, but they share the same feature: they sell into declines because a rule says so.
Episodes often cited in discussions of these mechanics include the volatility spike of February 2018, when products tied to short volatility collapsed, and the sharp declines of March 2020. In such episodes several mechanical sources of selling coincided.
Why the effect is lopsided
Negative gamma amplifies moves in both directions. A rally in a short-gamma market forces dealers to buy back their short hedges, and short-gamma rallies can be violent too. But on equity indices, two things make the downside version more common.
First, put open interest below the market is usually much heavier than call open interest above it, so the short gamma sits mostly beneath price. Second, the volatility relationship runs one way: volatility rises in declines and falls in rallies. In a rally, falling volatility shrinks put deltas and dealers buy back hedges, which helps; but it also shrinks the dealers' remaining short gamma, so the amplification fades as the market rises. In a decline, rising volatility deepens it.
Single stocks can be the exception. A name with heavy call open interest above the price and dealers short those calls can carry most of its short gamma overhead, and its amplified moves are more likely to be to the upside, which is the setup for a gamma squeeze rather than a selloff.
What this means for reading the map
The useful questions are where the zero gamma level sits relative to price, how much short gamma lies below it, and how close the heavy put strikes are. A market just above the zero gamma level with large put open interest a few percent below is one modest decline away from the regime where hedging amplifies.
None of this predicts a selloff. Negative gamma describes how a decline would behave if it started, not whether one will. Plenty of sessions spent below the zero gamma level end quietly. But when a decline does begin there, it has less to stop it.
Common questions
Does negative gamma mean the market will fall?
No. It means moves in either direction tend to be amplified. It describes the character of trading, not its direction.
Why is the downside usually worse in negative gamma?
Index put open interest below the market is typically heavier than call open interest above, and implied volatility rises in declines, which enlarges put deltas and adds to dealer selling.
Can negative gamma cause sharp rallies?
Yes. Short-gamma dealers must buy as price rises, and short-covering in a short-gamma market can be fast.
Short-gamma dealers must sell into declines to stay hedged, rising volatility enlarges that selling, and thinning liquidity magnifies its impact. Negative gamma does not cause selloffs, but it removes the brakes on the ones that start.
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