The S&P 500 has climbed around 12% in 2026, but that headline number masks a striking concentration of strength in just a handful of companies.
The S&P 500 Equal Weight Index, which assigns the same weight to every company in the index, has risen only about 9% over the same period.
That 3-percentage-point gap reveals how dependent the broader index’s performance has been on a small group of outperformers.
Around three-quarters of the stocks in the S&P 500 actually declined during September, with gains in a few heavyweight names masking widespread weakness across the rest of the index.
By one count, only about half of the companies that were in the index at the start of 2026 are currently in positive territory for the year.
Going into 2026, AI chip leader Nvidia (NVDA) made up about 7.8% of the Vanguard S&P 500 ETF (VOO), while Apple (AAPL) accounted for about 6.9%, according to the fund’s annual report, putting the two companies at nearly 15% of the fund combined.
Nvidia shares have climbed about 24% this year to roughly $231, while Apple has risen about 22%, meaning gains of that size from positions that large move the entire index considerably.
Micron Technology (MU) and Advanced Micro Devices (AMD) each started the year at just over half a percent of the fund, but their explosive gains made them outsized contributors to overall index performance.
Micron’s stock has nearly quadrupled in 2026, rising around 285%, while AMD has gained about 188%, with Micron’s revenue more than tripling in fiscal 2026 to around $133 billion.
Multiplying each stock’s starting weight by its gain, these four companies account for about 6 percentage points of the S&P 500’s 12% rise, meaning roughly $600 of every $1,200 earned on a $10,000 investment came from Nvidia, Apple, Micron, and AMD.
Not every major holding helped carry the index, as Tesla (TSLA) has dropped around 21% in 2026 and Microsoft (MSFT) has gained only about 6%, despite both beginning the year among the index’s 10 largest holdings.
Those laggards among the index’s giants likely prevented the gap between the cap-weighted and equal-weight versions of the S&P 500 from growing even wider than it already has.
For index fund investors, the mechanics of a cap-weighted fund worked as intended, automatically holding this year’s biggest winners in proportion to their growing size as their share prices rose.
The risk that comes with that structure is equally clear: if the chip stocks powering 2026’s gains give back ground, a cap-weighted S&P 500 fund could feel the impact more sharply than the average stock in the index would.
