• S&P 500 Index 3-month realized correlation stands at 4.0%—the 1st percentile of its 10-year history and a fraction of the 27.6% long-run average.
• Artificial Intelligence (AI)-linked names’ risk contributions run well above their capital weight.
• Low correlation is suppressing index volatility, not removing risk.
The calm in the S&P 500 is a correlation story, not a volatility story—and possibly the most overlooked current market theme. Realized correlation across index constituents sits at 4.0% today, lower than 99% of observations over the past decade. Individual stocks are volatile, but they are moving in different directions, and the offsets net out at the index level.
Beneath that surface, risk is concentrated. Market leadership rests on a narrow AI hardware and infrastructure complex—memory, semiconductors, equipment, and the newly listed SpaceX—and nearly every name in the cohort carries a larger share of dollar-weighted volatility than of market capitalization. Micron holds a 1.54% market weight but contributes 2.42% of dollar volatility. Intel and SanDisk each contribute nearly twice their capital weight in risk.
Correlations within the theme are almost unbelievable. Nvidia's 40-day correlation to other semiconductors (Broadcom and Micron) is near 0.50, while its correlation to the hyperscalers—its largest customers—has collapsed to roughly zero. The AI theme has sourced nearly three-quarters of the market’s earnings growth since the beginning of 2024, yet its components are practically uncorrelated in price movement.
We view current index volatility as understating the risk embedded in the cap-weighted portfolio. Investors measuring risk at the index level are underwriting a correlation assumption, not a volatility one—and the names carrying the most risk are the ones most likely to move together when the AI theme is tested.
Important Disclosures & Definitions
S&P 500 Index: widely regarded as the best single gauge of large-cap US equities. The index includes 500 leading companies and covers approximately 80% of available market capitalization.
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