Index implied volatility is, in a meaningful sense, a portfolio of single-name implied volatilities plus the implied correlation between them. When that decomposition starts to behave strangely, something is shifting in how the market expects stocks to move together.
The basic math
Index variance = weighted average of component variances + interaction terms based on correlation. If single-name vols are elevated but index vol is calm, the market is implying low correlation - stocks are expected to move on their own dynamics. If index vol is elevated but single-name vols are calm, the market is implying high correlation - everything moves together.
Why this is a signal
Macro regimes have characteristic correlation profiles. Stable bull markets show low correlation - winners and losers diverge. Crisis regimes show high correlation - everything sells off together. Watching the implied correlation embedded in options prices is watching the market's expected regime shift in real time.
How to read it without running the math yourself
- VIX low, single-name IVs rising: dispersion is increasing. Idiosyncratic trades getting easier. Macro getting easier.
- VIX rising, single-name IVs flat: correlation is increasing. Macro risk is building. Beta is becoming the only trade.
- VIX rising and single-name IVs rising together: broad vol regime shift. Everyone is bidding insurance.
Where dispersion trades show up in flow
Hedge funds running dispersion strategies will buy single-name vol and sell index vol when they expect dispersion to increase. The flow signature is concentrated single-name call/put buying paired with index put selling. When you see those legs at the same time, you are watching positioning for a less correlated regime.
Dispersion trades are how sophisticated funds bet on the shape of the next regime, not the level. The flow signature is subtle but readable once you know what to look for.