The S&P 500 in its current form dates to 1957: the index contains 500 companies and is weighted by market capitalization. Once viewed as broadly balanced, the index has become markedly top‑heavy. The combined weight of the top 10 companies was 19 percent in 1990, 29 percent in 2020, and is now about 43 percent.
The rise in concentration did not begin with artificial intelligence (AI), but AI’s rapid adoption has greatly accelerated it. Analysts warn that if the AI momentum were to falter suddenly — a realistic possibility given that prolonged success stories often experience temporary downturns or technological shifts — many of the top 10 stocks could struggle and the remaining hundreds of companies might not be able to offset a market dislocation.
Market echoes of the dot‑com era and the semiconductor rally
On Wall Street there is active debate about the possibility of an AI bubble bursting. One reason for the comparison to the dot‑com era is the record run in the PHLX Semiconductor Index, which tracks 30 representative U.S. semiconductor firms. Some chipmakers have risen by more than 100 percent this year, while the PHLX index itself is up a little over 80 percent year‑to‑date — a performance in the first 100 trading days last seen around 1995, before the dot‑com bubble.
It’s not only the semiconductor index: the equity risk premium versus bonds has nearly vanished over recent months. The market last witnessed a similar evaporation of the risk premium during the dot‑com bubble.
Why this matters: risks and counterarguments
According to investment theory and market tradition, equities should deliver higher long‑term returns than bonds because government securities are safer and more predictable. The current near‑zero equity risk premium suggests investors are pricing equity risk unusually low compared with historical norms.
Analysts and the market have tried to soothe concerns by noting that the largest components of the S&P 500 are highly profitable businesses with strong balance sheets, sustainable competitive advantages, and significant free cash flow. Still, the high concentration and sectoral — especially AI‑related — exposure increase the risk that a downturn concentrated among market leaders could have outsized effects on the broader index.
