The US stock market has continued to hit record levels in 2026. The S&P 500, Nasdaq Composite and Dow Jones Industrial Average have all gained more than 20% over the past 12 months, as of August 2026. A major reason behind the strong market rally has been the rapid growth of artificial intelligence (AI) stocks.
Investors have poured money into companies linked to AI because they expect the technology to drive strong growth in the coming years. But the same AI boom is now creating concerns about a possible AI bubble. Bank of America’s latest Global Fund Manager Survey found that fund managers see an AI bubble as the biggest tail risk facing the market right now, according to The Motley Fool.
S&P 500 faces mega-cap risk
One of the biggest concerns is how much of the S&P 500 is now controlled by a small group of mega-cap technology companies. The 10 largest companies make up around 40% of the S&P 500, according to data from S&P Global cited by The Motley Fool.
This is an unusually high level of concentration. The last time the S&P 500 was this concentrated was around 1965, according to S&P Global data. Even during the height of the dot-com bubble in March 2000, the 10 biggest companies made up only about 26% of the index. This means the S&P 500’s performance is now much more dependent on a small number of companies than it was during the dot-com bubble.
Why this concentration could be a problem
When the biggest companies perform well, they can push the entire S&P 500 higher. That has helped the index deliver strong returns during the recent AI boom. But the same concentration can become a major problem if these companies suddenly fall.
A sharp decline in just a few mega-cap stocks could put pressure on the entire S&P 500. Investors may therefore face greater market risk even if many smaller companies are performing well. In simple terms, the market has more eggs in fewer baskets.
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Nvidia, Apple, Alphabet, Microsoft and Amazon are at the centre of the risk
Nvidia, Apple, Alphabet, Microsoft and Amazon are currently the five largest stocks in the S&P 500, according to The Motley Fool. All five are making major investments in artificial intelligence, leaving the broader index increasingly exposed to the success of the AI boom. If demand for AI continues to grow, these companies could provide further support to the market.
The reverse is also true. Any concerns about the amount of money being spent on AI, the companies’ future growth or their high valuations could put pressure on their shares. Given their enormous weight in the S&P 500, a sharp decline in even a few of these stocks could drag the wider index lower.
The AI investment boom extends well beyond software and chatbots. Major technology companies are pouring billions into data centers and other infrastructure needed to support AI systems. Amazon, for example, spent nearly $100 billion on data centers during the first half of 2026 alone, according to The Motley Fool.
That level of spending shows how aggressively the biggest technology companies are positioning themselves for an AI-driven economy. But it also raises a crucial question for investors: Will these enormous investments eventually generate enough profits to justify their cost? If returns fail to meet expectations, companies could be forced to scale back spending, potentially putting pressure on the wider network of businesses benefiting from the AI boom.
The market may be showing another warning sign
The current situation is also raising concerns because stock valuations are extremely high. One measure investors use to study this is the S&P 500 Shiller CAPE ratio. CAPE stands for Cyclically Adjusted Price-to-Earnings ratio. It compares the S&P 500’s current price with its average inflation-adjusted earnings over the previous 10 years. A higher CAPE ratio generally means stocks are more expensive compared with their long-term earnings.
CAPE ratio is far above its historical average
Since the 1870s, the CAPE ratio has averaged around 17, according to The Motley Fool. The ratio rose above 30 before the Great Depression. It later reached an all-time high of about 44 just before the dot-com bubble burst. As of the report, the CAPE ratio is above 41. That puts the current market at levels that have historically been associated with very high valuations.
The 40-plus CAPE level has happened only twice
The CAPE ratio has remained above 40 since May 2026, according to The Motley Fool. This is only the second time in history that the ratio has stayed above that level. The first time happened in January 1999. The ratio continued rising after that and remained elevated until the dot-com bubble finally burst in March 2000.
The current situation does not prove that another crash is about to happen. But the historical comparison is raising concerns because today’s valuation levels look similar to one of the most expensive periods in stock market history.
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Does this mean an AI bubble will burst in 2026?
There is no way to know exactly what the stock market will do in the coming months. The high CAPE ratio and extreme concentration in mega-cap technology stocks are warning signs, but they are not crash predictions. The current market could continue rising if corporate earnings remain strong and AI produces the growth investors expect.
At the same time, investors should remember that market downturns are a normal part of investing. The CAPE ratio cannot tell investors exactly when a correction or bear market will happen. It can, however, suggest that many stocks may currently be priced at very high levels.
A market downturn does not mean investors will permanently lose money. The Motley Fool points out that the S&P 500 has produced more than 700% in total returns since the beginning of the dot-com bear market in March 2000. Many individual companies have performed even better over the same period. This shows why investors should think beyond short-term market crashes. Strong businesses can recover and continue growing even after major market downturns.
What investors should do now
The key lesson is not necessarily to sell everything because of fears about an AI bubble. Instead, investors should focus on quality companies with strong fundamentals. Businesses with healthy finances, strong competitive advantages and sustainable growth may have a better chance of surviving a market downturn.
Some highly valued technology stocks could struggle badly if the AI boom loses momentum. But companies with strong underlying businesses may be better positioned to recover over the long term. The Motley Fool therefore recommends focusing on healthy companies rather than simply chasing the hottest AI stocks.
The bigger market warning
The main warning for investors is that the S&P 500 has become unusually dependent on a handful of mega-cap companies. At the same time, many of those companies are heavily tied to the AI investment boom. The index is also trading at historically high valuation levels, with the CAPE ratio above 40.
High valuations, heavy concentration in a handful of companies and massive spending on AI could leave the market vulnerable if investor sentiment shifts. Nvidia, Apple, Microsoft, Amazon and Alphabet have been major forces behind the S&P 500’s gains, but their enormous size also means a decline in these stocks could have a disproportionate impact on the broader market.
Past market cycles do not mean a crash is inevitable in 2026, but they do offer a reminder that periods of sharp volatility can emerge quickly. For long-term investors, the better approach may be to focus on companies with strong businesses and solid fundamentals rather than trying to predict exactly when an AI-driven market bubble could burst.