What Past Sector Concentrations Tell Us About Today's Tech-Heavy S&P 500

The S&P 500’s 12-month rolling correlation with the Nasdaq-100 reached an all-time high of 0.98 in March 2026, as Information Technology now accounts for nearly 40% of the index. Since 2020, the S&P 500’s correlation with its equal-weighted counterpart has trended lower to around 0.8, signaling a historic divergence between the market-cap index and the broader market. Software platforms, cloud infrastructure and artificial intelligence companies now sit at the top of the weightings. By Dr. Mark Shore When the S&P 500 was introduced in 1957, industrials accounted for 85% of its weighting. Factories, steel mills, and oil rigs defined the American economy, and the index reflected that reality. Nearly seven decades later, the composition of that same index tells a radically different story. Software platforms, cloud infrastructure and artificial intelligence companies now sit at the top of the weightings. A small cluster of massively valued technology companies - widely known as the “Magnificent Seven” (Mag 7) - has fundamentally reshaped what the S&P 500 actually represents. That raises a question worth examining closely: has this concentration altered how the major U.S. equity benchmarks relate to one another? The Weight of a Few The S&P 500 is a market capitalization-weighted index. In practice, that means the largest companies by market value exert the most influence over the index’s daily movement, and right now, that influence is highly concentrated. Driven by the Mag 7 and the broader AI investment boom, the Information Technology (IT) sector’s share of the S&P 500 has surged from just under 7% in 1990 to nearly 40% today. In other words, a single sector now accounts for nearly six times the weight it carried just over three decades ago, and it exceeds the combined weight of most other sectors in the index. Moving in Lockstep: S&P 500 and the Nasdaq-100 This concentration means the S&P 500 is increasingly behaving like the Nasdaq-100, an index historically associated with high-growth technology exposure. From 1995 through approximately 2009, the 6-month and 12-month rolling correlations of daily returns between the two indexes climbed steadily - from around 0.6 to 0.96 - before leveling off at a relatively consistent 0.9. Then, beginning around 2017, correlations began trending even higher. By March 2026, the 12-month rolling correlation between the S&P 500 and the Nasdaq-100 reached 0.98 - an all-time high. These two benchmarks, once meaningfully distinct, are now moving in near-perfect lockstep. Drifting Apart: S&P 500 and Its Equal-Weighted Counterpart While the S&P 500 and Nasdaq-100 converge, the S&P 500 is simultaneously pulling away from its own equal-weighted version, an index where every constituent carries the same weight regardless of market cap. For years, the rolling correlation of daily returns between the S&P 500 and the S&P 500 Equal Weighted index regularly sat above 0.95, reflecting a market where broad participation drove returns. Since 2020, that relationship has deteriorated. The correlation has trended lower, often hovering around 0.8 for both 6-month and 12-month rolling periods, with notably wide and volatile swings. The widening gap between the lightest and heaviest sector weights in the S&P 500 is a key reason, and as concentration increases, the market-cap index and the equal-weighted index tell increasingly different stories about market performance. What History Tells Us Extreme sector concentration in the S&P 500 is not without precedent, though today’s situation is arguably unique in its scale and persistence. The energy sector surged to nearly 28% of the S&P 500 in 1980 in the wake of the oil price shock, then retreated steadily to roughly 3% today. The IT sector climbed from 7% in 1990 to approximately 33% at the peak of the dot-com bubble in March 2000, only to revert to mid-teen weightings in the early 2000s as the.