Investors today face a landscape marked by significant index concentration, where a relatively small number of companies dominate major stock market benchmarks. While this phenomenon has raised concerns about portfolio diversification and potential risks, market experts advise that historical patterns offer some reassurance for long-term investors.

In the United States, the 10 largest companies within the S&P 500 make up nearly 40 percent of the index, with nine of those firms clustered in technology or technology-adjacent sectors. Canada’s S&P/TSX Composite also shows notable concentration, as its top 10 companies—primarily in financials, materials, and energy—account for roughly 38 percent of the index’s value. Additionally, global diversification may be less effective than assumed since the U.S. constitutes almost three-quarters of the MSCI World Index, which tracks over 1,200 stocks across 23 developed markets.

Toronto-based Picton Investments recently surveyed Canadian investors, revealing that 83 percent feel confident about the diversification of their portfolios. However, just 57 percent are familiar with the concept of index concentration, a gap that raises concerns. Pawan Vatvani, head of portfolio construction at Picton’s 2nd Engine unit, cautioned that investors—both retail and institutional—often overestimate diversification simply by holding multiple funds. He explained that owning several equity funds can result in “paper-based” diversification where underlying risk exposures overlap significantly.

The strong performance of major indexes amid concentrated holdings has contributed to the widespread appeal of technology stocks and artificial intelligence (AI) companies. In the first half of 2026, 67 percent of actively managed large-cap U.S. equity funds underperformed the S&P 500, and over the past five years, nearly 90 percent lagged the benchmark, according to S&P Global data. This has reinforced investor confidence in indexing strategies despite warning signs beneath the surface.

While broader index values have risen, more than 40 percent of S&P 500 components fell during the year, illustrating that gains are not uniform across all stocks in the index. Steve Sosnick, chief strategist at Interactive Brokers, noted ongoing investor enthusiasm for technology names, describing recent market dips as buying opportunities propelled by optimism around AI’s “limitless potential.” He compared AI-related capital expenditure, estimated at US$800 billion in 2026 and expected to reach US$1 trillion in 2027, with the scale of the 2008 U.S. financial crisis bailouts, emphasizing the magnitude of investment driving sector gains.

However, Sosnick cautioned that much institutional investor capital is already deployed in technology and AI stocks, potentially limiting market upside during upcoming earnings reports—even if results are strong. This concentration could also intensify volatility if market sentiment shifts.

Despite these risks, some experts assert that historical experience with concentrated markets reduces cause for alarm. Benjamin Felix, chief investment officer at Montreal-based PWL Capital, said the relationship between market concentration and future returns in the U.S. has been "very slightly negative" but statistically insignificant. Felix cited Nortel Networks as an example: once representing over a third of the Canadian market, Nortel collapsed in 2009, yet Canada's broader market outperformed the U.S. through the following tech crash and downturn.

As investors navigate an environment characterized by concentrated indexes and rapid technological change, specialists emphasize the importance of understanding underlying risks while also considering the broader lessons history provides.