75% of S&P 500 Stocks Fell in September as Index Clung to Gains

Three-quarters of the S&P 500's component stocks declined in September, even as the benchmark index itself posted a slight gain for the month, according to MarketWatch's Philip van Doorn.

The split is stark. The S&P 500 sat roughly 1% below its record high in early trading on Wednesday, Sept. 30, yet the broad market beneath that headline number was falling. Van Doorn's tally, published that morning, put the share of declining components at 75%.

That divergence has become a central talking point on Wall Street. The index's resilience, Van Doorn reports, rests on its heavy top-weighting: a small group of large-cap stocks carries enough index influence to keep the S&P 500 near its peak while the majority of its members lose ground.

The concern among market watchers is straightforward. If three out of four stocks in the index are already in retreat, the benchmark's proximity to a record high may say more about the concentration of capital in a handful of mega-caps than about the health of the broader equity market. Analysts quoted across the piece frame the question as one of sustainability — whether index-level strength can hold when the underlying market is shrinking in breadth.

The numbers explain the mechanics. In a cap-weighted index like the S&P 500, the largest companies dominate the calculation. When those names advance, the index can rise even as most constituents fall. September delivered a textbook case: the index edged higher while 75% of its members declined, a gap between the headline number and the average stock that investors rarely see laid out so clearly.

Van Doorn's analysis, part of his Deep Dive column for MarketWatch, lists the 15 worst performers among S&P 500 components for the month — a roster behind the publication's subscription wall, but one that underscores where the damage concentrated. The companies named among the referenced symbols in the piece include TransUnion (TRU), down 3.94% in the session cited, alongside Fannie Mae (FNMA) and Freddie Mac (FMCC) preferred listings. Nvidia (NVDA), up 1.72% on the day referenced, illustrates the other side of the trade: the mega-cap strength that kept the index afloat while the majority of stocks sank.

The episode carries a broader lesson for portfolio construction. Investors holding cap-weighted index funds were largely insulated from September's drawdown in the average stock, because their exposure tracked the giants at the top. Investors holding equal-weighted or actively selected portfolios absorbed the full force of a month in which most stocks fell.

It also raises a timing question that Van Doorn's data makes concrete. An index 1% from its record, with 75% of members declining, has historically been a pattern worth watching: either the laggards recover and breadth improves, or the leaders eventually give way and the index follows the market down. September 2026 delivered the setup. The fourth quarter will show which force wins.