Four Stocks Drive Half of S&P 500's 2026 Gain

The S&P 500 index has risen around 12% in 2026, but its average stock hasn't kept up. The S&P 500 Equal Weight Index, which gives each company an equal weight, has gained about 9%. In September, the two indices split apart, with the equal-weight index falling 5% while the market-value-weighted sibling dipped only 0.5%

Around three-quarters of the stocks in the S&P 500 dropped over the month. Nvidia and Apple, which made up nearly 15% of Vanguard's S&P 500 fund at the start of 2026, hid this weakness with their gains.

Micron Technology and Advanced Micro Devices, both chipmakers, saw their shares rise dramatically. Micron's stock has almost quadrupled in 2026, while AMD's has gained about 188%. Their weight in the index has increased significantly, now making up a larger portion of the fund.

The success of these four stocks accounts for nearly half of the S&P 500's gain this year. If they give some of it back, an S&P 500 fund might feel it more than the average stock would.