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The S&P 500 and its equal-weight sibling are supposed to track the same 500 companies. Same stocks, same universe, same American economy. Yet the two indices are behaving less alike than at any point in recorded history.
The one-year correlation between the S&P Equal Weight Index (SPW) and the cap-weighted S&P 500 (SPX) has fallen to 79%, a record low. For context, a correlation of 1.0 means two things move in perfect lockstep. A reading of 0.79 between two indices holding the exact same companies is, to put it politely, unusual.
What’s actually happening under the hood
The S&P 500 and the S&P Equal Weight Index contain identical stocks. The difference is purely mathematical: how much each stock matters.
In the traditional cap-weighted S&P 500, a company’s influence scales with its market value. The top 10 companies now account for roughly 40% of the entire index’s weight. That means when Apple or Nvidia has a big day, the index moves whether or not the other 490 stocks participate.
The Equal Weight Index assigns a fixed 0.2% allocation to each of its 500 members, rebalancing quarterly to maintain that democracy.
The concentration problem in plain numbers
This concentration has been building since around 2020, driven largely by mega-cap technology and AI stocks. The rolling 12-month correlations between SPW and SPX have been hovering around 0.80 with increasing volatility throughout this period, steadily drifting lower as a narrower set of names dominated returns.
Prior to 2023, the Equal Weight Index actually outperformed the cap-weighted S&P 500 by approximately 1.05% annually. That trend has reversed in recent years as mega-cap dominance intensified.
Why this matters for your portfolio
Many investors assume that buying an S&P 500 index fund gives them broad exposure to the American economy. At current concentration levels, that assumption deserves scrutiny. You’re getting heavy exposure to a small number of mega-cap names, with a long tail of companies that barely move the needle on your returns.
Equal-weight strategies offer a structural hedge against this concentration. By treating every stock equally, they capture gains from mid-cap and smaller large-cap companies that the headline index essentially ignores. During periods of low correlation between the two approaches, holding both can provide genuine diversification benefits within the same stock universe.
There are already hints of rotation emerging in early 2026, suggesting that market dynamics may be starting to shift. Historically, periods of extreme concentration in the S&P 500 have eventually given way to broader market participation, and when that reversal occurs, equal-weight strategies tend to outperform.
A 40% allocation to 10 companies means a significant earnings miss, regulatory action, or sector-specific downturn in technology and AI could drag the entire cap-weighted index in ways the equal-weight version would largely shrug off. The correlation gap is a measure of how asymmetric that risk has become.
Disclosure: This article was edited by Editorial Team. For more information on how we create and review content, see our Editorial Policy.
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