About three-quarters of the companies in the S&P 500 fell during September even as the headline index stayed close to its record, according to separate reports from CNBC and MarketWatch. The contrast showed that the benchmark’s level gave a much stronger impression than the experience of most individual stocks. MarketWatch said the S&P 500 was about 1% below its record near month-end, while its components were broadly weaker beneath the surface.
The divergence reflected the S&P 500’s market-capitalization weighting, which gives the largest companies an outsized influence on the index. MarketWatch reported that eight of the benchmark’s 11 sectors declined during September. Large technology shares accounted for many of the stronger performers and helped offset losses elsewhere. CNBC likewise described the month as poor for most individual S&P 500 stocks despite the relative strength visible at the index level.
The breadth weakness extended beyond one difficult month. MarketWatch said 44% of S&P 500 members were down for the year even as the index retained a 12.4% year-to-date gain. Nvidia illustrated how the largest constituents can shape the benchmark: the chipmaker represented 8.3% of the SPDR S&P 500 ETF Trust and had advanced 23.6% in 2026. A gain of that size in a heavily weighted company can counter declines across a much larger number of smaller members.
Several of September’s steepest losses were tied to company-specific developments. MarketWatch identified Fair Isaac as the month’s weakest S&P 500 member, with a 27% decline after mortgage agencies announced plans to use VantageScore credit data, creating a potential challenge to FICO’s established position. Equifax and TransUnion also fell sharply. The declines reduced forward price-to-earnings ratios across a number of sectors, even as corporate earnings remained an important support for the broader index.
What it means for traders: the September figures show that the capitalization-weighted S&P 500 did not fully reflect conditions across the wider US equity market. Concentrated gains in a handful of megacaps can keep the benchmark near a high while equal-weighted exposure and most sectors lag. That narrow leadership can continue if the largest companies maintain earnings momentum, but it also leaves the index more sensitive to reversals in those heavily weighted shares. The breadth data describe concentration rather than establishing a directional forecast.
The next point to watch is whether participation broadens as the fourth quarter begins. A larger share of advancing stocks and stronger performance across more sectors would narrow the gap between the index and its constituents. If large technology companies continue to account for most of the benchmark’s strength while a majority of members remain weak, the divergence highlighted by CNBC and MarketWatch will persist. Corporate results and changes in Treasury yields will also help determine whether earnings support can offset the pressure from higher discount rates.