AI-Linked Stocks Reach 49% of S&P 500, Reviving Historical Bubble Comparisons

AI-related stocks now reportedly account for approximately 49% of the S&P 500's market capitalization, based on a basket of 41 companies classified by JPMorgan.
The concentration has prompted comparisons with previous market booms, including railroads in the 1880s, the Roaring Twenties, the Nifty Fifty, Japan's late-1980s equity bubble and the dot-com era.
Those episodes were followed by severe market declines, with the Nasdaq falling roughly 78% after its 2000 peak and Japan's Nikkei losing more than 80% from its 1989 high.
Bianco Research also suggests AI concentration could hypothetically exceed 50% if SpaceX were included. However, SpaceX is not an S&P 500 constituent, historical comparisons depend on classification methods, and market concentration alone does not establish that another crash is inevitable.