AI Bubble Warning: S&P 500 Reaches New Heights Amid Rising Concerns of Overvaluation

The stock market has been on a tear in recent years, but experts are warning that an AI bubble may be forming. The S&P 500 has skyrocketed by more than 20% over the past 12 months, as of August 2026, with many of the gains coming from tech stocks.

The concentration of big tech companies in the index is a concern, with the top 10 holdings accounting for around 40% of the S&P 500. This level of concentration hasn't been seen since 1965, and even at the peak of the dot-com bubble in March 2000, the index's top 10 holdings were weighted at only around 26%.

The majority of the S&P 500's largest companies are heavily invested in AI, including Nvidia, Apple, Alphabet, Microsoft, and Amazon. These companies have taken big swings on AI, with Amazon alone spending nearly $100 billion on data centers in the first half of 2026.