Mid-2026 markets rise, but gains are concentrated in big AI-linked stocks

In short: Stocks and bonds have done well in 2026 so far, but a small number of very large US tech and AI-related companies are driving a big share of stock market gains.

What's going on

Investors have had a good first half of 2026. Major stock markets are up, and bonds are also delivering solid returns. Several market outlooks still flag a growing concern, the stock market is becoming more concentrated.

“Concentrated” means fewer companies make up more of the market’s total value, like a sports team that relies on just a few star players. iShares said the 10 largest companies in the S&P 500 now account for more than 40% of the index’s total market value. The firm also said AI-related gains have helped push US stock indexes to new highs, while increasing the risk that many stocks move together.

Other firms see a similar pattern. Cambridge Associates said concentration risk in US stocks has increased a lot, and it cited this as one reason to slightly favor stocks outside the United States. Russell Investments added that investment activity is also clustering around a handful of AI leaders, and that upcoming IPOs (when a private company starts selling shares to the public) could add to that crowding instead of easing it.

What to watch

Diversifying, which means spreading money across different types of investments, may be harder than it used to be. iShares noted that stocks and bonds do not always move in opposite directions anymore, which can reduce bonds’ role as a buffer when stocks fall. If the biggest tech and AI-linked companies stumble, investors will be watching whether the rest of the market can hold up on its own.

Source: NYTimes