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The benchmark finished the third quarter up roughly 2% and only about 2% below its 52-week high, according to Nasdaq market data . But the internal picture is far weaker: 83% of S&P 500 members are more than 10% below their individual 52-week highs, while 42% are down at least 20%. September exposed the divide even more clearly. The headline S&P 500 fell only about 0.5%, yet roughly 78% of its stocks declined during the month. Mega-Caps Are Hiding the Weakness The reason is simple: the S&P 500 is weighted by market capitalization. Large companies therefore have far more influence over the index than smaller constituents. When Nvidia, Microsoft, Alphabet, Meta and other mega-cap names hold up, the benchmark can remain close to record territory even while hundreds of stocks are falling. That gap is visible in the equal-weight S&P 500 , which gives every company roughly the same influence. The New York Stock Exchange says the normal S&P 500 gained about 2% in Q3, while the equal-weight version fell around 2% and recorded seven consecutive weekly declines. something that has happened only twice previously. Higher Yields Are Hitting the Rest of the Market The weakness has coincided with a sharp repricing in bonds. The 10-year Treasury yield climbed 54 basis points in September to 5.29%, its largest monthly increase in four years. Higher yields raise discount rates and borrowing costs, which hurts smaller companies, rate-sensitive sectors and businesses that depend more heavily on external financing. Nine of 11 large-cap sectors finished September lower, while the Russell 2000 dropped 5.3%. Technology and communications were the major exceptions, helping explain why the market-cap-weighted indexes held up much better than the broader market. That concentration was already visible before September. The S&P 500’s ten largest companies accounted for 37.8% of the index, with Nvidia alone representing 8.1%. The growing influence of AI-linked giants means a relatively small group of companies can increasingly determine the direction of the entire benchmark.