The S&P 500 has delivered strong headline returns in 2026, but a closer examination of what is driving those returns1 reveals a portfolio risk that is mounting beneath the surface. In a new analysis from DoubleLine Capital, Portfolio Manager Eric Dhall and Strategist Ryan Kimmel lay out the case that concentration — not diversification — now defines U.S. equity exposure for most investors, and that the tools to address it are already available.
A Market Running on a Handful of Names
Through the first nine months of 2026, the S&P 500 returned 12.7%. The result looks healthy at the index level. Strip away the top performers, and a very different picture emerges. Five stocks accounted for 52% of that return. The top 10 accounted for 65%. The top 20 accounted for 87%. As Dhall and Kimmel note, this represents "a narrowness of market breadth not seen since the late 1990s."
The concentration is not limited to individual names. As of September 30, the information technology sector alone represented 40% of the S&P 500, while tech generated nearly half of the index's return over the prior three years. The five largest stocks represent 28% of the index by weight; the top 20 represent nearly half.
This is the central diagnostic of the Dhall-Kimmel analysis, and it frames every argument that follows. A portfolio that mirrors the S&P 500 is not broadly diversified. It is a leveraged bet on a narrow band of megacap technology companies.
Concentration Risk Is Not the Same as Valuation Risk — But Both Are Present
Dhall and Kimmel are careful to distinguish the current environment from the late-1990s dot-com bubble. The distinction matters. During that period, the S&P 500 climbed 98% between March 1997 and March 2000, while forward earnings rose only 32%, pushing forward price-to-earnings multiples to 25.5x. Today's three-year run looks meaningfully different: the index climbed 78% while forward earnings kept pace at 66%, with forward P/E multiples at 21.2x.
The conclusion drawn: "Market concentration alone does not necessarily signal an impending correction." The fundamentals behind today's market leaders are, on the whole, real.
But fundamental strength does not eliminate concentration risk. By the end of 2025, the 10 largest S&P 500 companies accounted for more than half of the index's volatility. Their aggregate volatility was 1.5 times that of the S&P 500 overall. Larger weights, greater single-stock volatility, and higher correlation among tech-related companies mean the largest constituents "now potentially influence portfolio risk even more than their substantial index weights suggest."
Valuation is also not neutral. The S&P 500 CAPE ratio stands near levels last observed at the dot-com peak. History shows a meaningful negative relationship between starting valuations and subsequent long-term real returns. The current tech sector CAPE ratio sits in a range "historically associated with muted forward real returns." None of this predicts when leadership will shift. As the authors acknowledge, equity diversification is "not about an exercise (likely futile) in calling the exact turning point in market leadership."
Two Paths to Genuine Diversification
DoubleLine offers two strategies designed to address the concentration problem from different angles.
The first is the CAPE strategy, based on the Shiller Barclays CAPE U.S. Sector Total Return Index. The strategy is built to diverge from cap-weighted index construction. It seeks exposure to four inexpensive S&P 500 sectors based on relative cyclically adjusted price-to-earnings ratios, while using momentum to avoid value traps. Since September 2002, the index has generated a 12.39% annualized return versus 11.48% for the S&P 500, with lower annualized volatility and a higher Sharpe ratio over that period.
The recent underperformance of CAPE relative to the S&P 500 — driven by the AI-fueled surge in tech — is addressed directly. The authors frame the divergence as a feature, not a failure: "The recent underperformance of CAPE has coincided with precisely the type of protracted momentum market its valuation discipline was designed not to chase but to avoid." That creates short-run tracking error, but does not contradict the long-term thesis.
The second strategy is based on the Barclays Fortune 500 Equal Weighted Total Return Index. Where CAPE addresses concentration at the sector level, the Barclays Fortune 500 strategy addresses it "at the company level." The Fortune 500 universe is defined by revenue, not market capitalization, and each company is equally weighted. Technology represents 10% of the Barclays Fortune 500 versus 40% of the S&P 500. Since April 1996, the index has generated an 11.77% annualized return versus 10.39% for the S&P 500, with a higher Sharpe ratio.
Why Advisors Should Not Wait
The closing argument in the Dhall-Kimmel analysis is its most direct. The strategies are not replacements for cap-weighted exposure. They are strategic complements. "CAPE diversifies valuation risk; Barclays Fortune 500 diversifies single-company concentration risk." Used individually or together, they reduce dependence on a narrow group of market leaders.
The warning is equally plain: "Investors should not wait for concentration to unwind before diversifying." After years of rising concentration, investors in cap-weighted large-cap indices are "increasingly making the same bet on a handful of megacaps whether they intend to or not."
Five Key Takeaways for Advisors and Investors
- Passive S&P 500 exposure is no longer structurally diversified. With the top 20 stocks accounting for 87% of YTD returns through September 2026, a cap-weighted index allocation is a high-conviction tech bet, whether investors intend it to be or not.
- Concentration risk and valuation risk are distinct but co-present. Unlike the late 1990s, today's megacap leaders have genuine earnings growth behind them. But the weight and volatility of those names means portfolio risk now exceeds what index weights alone suggest.
- CAPE underperformance reflects discipline, not a broken thesis. The strategy's avoidance of expensive tech over 2023 to 2026 produced short-run tracking error, but its long-run record on risk-adjusted returns remains intact.
- Revenue-weighted diversification offers a structurally different exposure. The Fortune 500 equal-weight approach reduces tech concentration from 40% to 10% while maintaining full U.S. large-cap equity exposure.
- The time to diversify is before leadership shifts, not after. Calling the turning point in market leadership is, in the authors' own assessment, likely futile. The value of diversification strategies lies precisely in being positioned ahead of that transition.
Footnote:
1 Dhall, Eric S., and Ryan Kimmel. "Diversifying U.S. Equity Exposure in a Concentrated Market." DoubleLine Capital, Oct. 2026, https://doubleline.com/wp-content/uploads/DoubleLine_Diversifying-Equity-Exposure_October-2026.pdf.
