How the S&P 500 Stock Index Became So Skewed to Tech and A.I.

Nvidia, the chipmaker that became the world’s most valuable public company two years ago, was alone worth more than $4.75 trillion as of Thursday morning. Its value, or market capitalization, is more than double the combined worth of all the companies in the energy sector, including oil giants like Exxon Mobil and Chevron.

The chipmaker’s market cap has swelled so much recently, it is now 20 percent greater than the sum of all of the companies in the materials, utilities and real estate sectors combined.

What unifies these giant tech companies is artificial intelligence. Nvidia makes the hardware that powers it; Microsoft, Apple and others have been making big bets on products that people can use in their everyday lives.

But as worries grow over lavish spending on A.I., as well as the technology’s potential to disrupt large swaths of the economy, the outsize influence that these companies exert over markets has raised alarms. They can mask underlying risks in other parts of the index. And if a handful of these giants falter, it could mean widespread damage to investors’ portfolios and retirement funds in ways that could ripple more broadly across the economy.

The dynamic has drawn comparisons to past crises, notably the dot-com bubble. Tech companies also made up a large share of the stock index then — though not as much as today, and many were not nearly as profitable, if they made money at all.

To understand how abnormal and worrisome this moment might be, The New York Times analyzed data from S&P Dow Jones Indices that compiled the market values of the companies in the S&P 500 in December 1999 and August 2007. Each date was chosen roughly three months before a downturn to capture the weighted breakdown of the index before crises fully took hold and values fell.

The companies that make up the index have periodically cycled in and out, and the sectors were reclassified over the last two decades. But even after factoring in those changes, the picture that emerges is a market that is becoming increasingly one-sided.

In December 1999, the tech sector made up 26 percent of the total.

In August 2007, just before the Great Recession, it was only 14 percent.

Today, tech is worth a third of the market, as other vital sectors, such as energy and those that include manufacturing, have shrunk.

Since then, the huge growth of the internet, social media and other technologies propelled the economy.

Now, never has so much of the market been concentrated in so few companies. The top 10 make up almost 40 percent of the S&P 500.

With greater concentration of wealth comes greater risk. When so much money has accumulated in just a handful of companies, stock trading can be more volatile and susceptible to large swings. One day after Nvidia posted a huge profit for its most recent quarter, its stock price paradoxically fell by 5.5 percent. So far in 2026, more than a fifth of the stocks in the S&P 500 have moved by 20 percent or more. Companies and industries that are seen as particularly prone to disruption by A.I. have been hard hit.

The volatility can be compounded as everyone reorients their businesses around A.I, or in response to it.

The artificial intelligence boom has touched every corner of the economy. As data centers proliferate to support massive computation, the utilities sector has seen huge growth, fueled by the energy demands of the grid. In 2025, companies like NextEra and Exelon saw their valuations surge.

The industrials sector, too, has undergone a notable shift. General Electric was its undisputed heavyweight in 1999 and 2007, but the recent explosion in data center construction has evened out growth in the sector. GE still leads today, but Caterpillar is a very close second. Caterpillar, which is often associated with construction, has seen a spike in sales of its turbines and power-generation equipment, which are used in data centers.

One large difference between the big tech companies now and their counterparts during the dot-com boom is that many now earn money. A lot of the well-known names in the late 1990s, including Pets.com, had soaring valuations and little revenue, which meant that when the bubble popped, many companies quickly collapsed.

Nvidia, Apple, Alphabet and others generate hundreds of billions of dollars in revenue each year.

And many of the biggest players in artificial intelligence these days are private companies. OpenAI, Anthropic and SpaceX are expected to go public later this year, which could further tilt the market dynamic toward tech and A.I.