The 15 largest stocks in the S&P 500 now account for a record 46.7% of the index’s total market value, compared to just 13.4% for the top three in the mid-1980s, according to October 2026 data. The three largest market caps (NVIDIA, Apple, Alphabet) together exceed 21% of the index, with NVIDIA alone contributing approximately 13% of the S&P 500’s rally between March and October 2026. Major tech companies (Microsoft, Amazon) each surpass $2.8 trillion in market cap, driven by massive hyperscaler investments in AI infrastructure. This unprecedented concentration means that holding an S&P 500 index fund essentially amounts to a concentrated bet on a handful of tech giants. Nearly half of the top 15 companies in 2026 were not in the top 15 in 2022, illustrating the rapid redistribution within the index.
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