A Bank of America chart shows that the top ten AI stocks now account for approximately 40% of the total market capitalization of the US stock market, approaching the 40% of the "Nifty Fifty" in 1972 and the 41% during the tech bubble of 2000, but still lower than the 63% held by railroad stocks in the 19th century.
High concentration alone cannot prove an impending bubble burst, but it does mean that index movements, passive fund performance, and market sentiment are increasingly dependent on a few companies. AI industry demand and profitability can continue to grow, but if earnings growth, capital expenditure returns, or valuation digestion speed fall short of expectations, crowded holdings will amplify corrections.
Note that the 44% figure for Japan in 1989 uses the global stock market ratio, while most other data uses the US stock market ratio; there are differences in statistical methods for horizontal comparison.
