Investors are facing a period of historically high concentration in the S&P 500 Index headed into 2026, with a small number of mega-cap technology and AI-related companies dominating the index's performance and risk.

That's leading more investment managers to advise clients as part of an annual portfolio review process to pay extra attention to opportunities to broaden holdings within the U.S. market, and across both value and overseas stocks.

"The big theme for us is making sure we have resiliency built into the portfolio and the way we are going about that is diversification," Kathmere Capital CIO Nick Ruder said on CNBC's "ETF Edge" on Monday.

He expressed concern that investors remain too concentrated in the " Magnificent 7" stocks, which currently make up about 35%

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