As the rally approaches its fourth anniversary on October 12, the S&P 500 remains near record highs, making this the sixth-strongest U.S. bull market since World War II.
However, the 10 largest companies now represent approximately 40% of the benchmark's market capitalization, highlighting Wall Street's growing dependence on artificial intelligence.
S&P 500's Four-Year Bull Market Has Delivered 117%
The rally began after U.S. stocks bottomed in October 2022, when aggressive Federal Reserve tightening and recession fears pressured valuations.
Since then, enthusiasm for artificial intelligence, improving corporate profits and resilient economic growth have helped push equities substantially higher.
The S&P 500 has gained more than 13% in 2026 alone, despite rising oil prices and Treasury yields above 5%.
AI-linked companies have been central to those gains.
Nvidia benefits from demand for AI accelerators, while Microsoft, Alphabet, Amazon and Meta continue investing heavily in data centers and computing infrastructure.
The AI earnings boom has also strengthened expectations for corporate profitability.
Just 10 Companies Now Represent 40% of the S&P 500
The S&P 500 is weighted by market capitalization, meaning larger companies have more influence on index movements.
According to S&P Dow Jones Indices, the 10 largest constituents represented 37.8% of the index at the end of August.
By September 25, that concentration had risen to approximately 40.4%, based on holdings data from an S&P 500 tracking ETF.
Nvidia alone represented roughly 8.1%, followed by major technology companies including Apple and Microsoft.
That concentration creates an important distinction between index performance and the experience of individual investors.
Coinpaper previously reported that 70% of S&P 500 stocks were trading at least 10% below their recent highs, even as the benchmark reached records.
