The gap between business growth and stock performance is wider than investors think. Research Affiliates’ Rob Arnott explains why index membership may be the real driver of mega-cap dominance.
The Market Crowns Winners Before the Race Is Over
One of the most durable myths in investing is that the largest companies grow the fastest. The logic seems intuitive: scale, visibility, institutional backing, and benchmark dominance should compound into superior returns. But Rob Arnott, Partner and Chair at Research Affiliates, argues that the historical record tells a very different story, and that the implications for portfolio construction are both urgent and underappreciated.
In a July 2026 paper published as part of the CFA Institute Research Foundation Series1, Arnott and co-author Lillian Wu present 35 years of evidence drawn from CRSP and Compustat data comparing two mechanically defined portfolios: the TrueCap 500, the 500 largest U.S. companies by market cap, and the Next 500, the firms ranked immediately below. The findings are striking. The Next 500 grew operating cash flow at roughly 9% annually versus approximately 6.5% for the top tier, compounding to more than double the cash flow base by 2025. Yet the return advantage for the Next 500 was only about 0.6% per year, with meaningfully higher volatility. In other words, the faster-growing businesses did not deliver proportionally superior returns. The slower-growing giants kept pace not by outgrowing their peers, but by commanding ever-higher valuation multiples.
"The giants did not keep up because they grew faster," Arnott writes. "They kept up because their comparable returns coincided with investors paying more and more for each dollar of their fundamentals."
This is the core tension the paper is built around. Valuation expansion, not business growth, has driven the relative outperformance of the largest-cap cohort over the full sample period.
When the Multiple Becomes the Message
The valuation premium of the top 500 relative to the Next 500 did not exist in 1989. It was actually a discount. Over the subsequent 35 years it averaged roughly 17%, and by mid-2025 it had reached an unprecedented 80% on a price-to-cash-flow basis. Critically, that expansion has not been matched by fundamental improvement.
Exhibit 2 in the paper makes the relationship visible. Relative investor wealth has tracked relative valuation multiples far more closely than relative cash flow growth. Large-cap outperformance has coincided with expanding premiums; relative underperformance has aligned with compression. The red line, representing relative fundamentals, has told a consistently modest story. The green line, relative price-to-cash-flow, has done most of the storytelling.
Arnott's conclusion is direct: "A high multiple can masquerade as growth. Index membership can masquerade as merit. Popularity can masquerade as progress. Cap weighting reinforces the loop."
The Structural Privileges of Inclusion
Why has this loop persisted? The paper points to the mechanics of modern index investing as the key mechanism. As passive ownership has grown from roughly 3% of U.S. market cap in 1990 to more than 50% by 2025, inclusion in the largest-cap cohort has become a structural tailwind in its own right. The S&P 500 now spans approximately 80% of U.S. stock market capitalization and passively owns around one-quarter of each member company's market value. Every new dollar flowing into index funds mechanically reinforces the price advantage of members over non-members. Rising prices create larger index weights; larger weights attract more capital; more capital drives prices higher still. The process is self-reinforcing and, Arnott argues, increasingly disconnected from the fundamental performance of the underlying businesses.
"Rising prices confirm rising growth expectations," Arnott observes. "In an efficient market, these rising expectations would be just as likely to underestimate as to overestimate future growth."
The AI Analogy
The paper draws an explicit parallel to the current AI moment. NVIDIA represents the consensus choice. But Arnott suggests the better opportunity may lie one step removed from the headline, in the power management systems, thermal infrastructure, substations, grid construction, and end users of AI, businesses that participate in the same growth story at a very different price. Exhibit 3 supports the broader principle: portfolios constructed using realized fundamental growth, measured by subsequent growth in sales, profits, or R&D spending, have outperformed portfolios constructed using traditional price-based growth definitions over the full sample.
Five Key Takeaways for Advisors and Investors
- Business growth and stock performance are not the same thing. The fastest-growing companies by fundamentals have not historically delivered the highest returns when the market has already priced that growth in.
- Valuation expansion has been the primary driver of large-cap outperformance, not superior earnings growth. The 80% price-to-cash-flow premium of the top 500 versus the Next 500 represents a historically unprecedented divergence from fundamentals.
- Index membership is now an investment factor in its own right. Passive flows create structural demand for index constituents independent of business quality, inflating multiples and reinforcing momentum.
- Fundamental growth, measured by realized sales, profits, and R&D expansion, has been a more reliable forecaster of future returns than valuation multiples. The market's definition of "growth" investing warrants scrutiny.
- In technology-driven investment cycles, the most richly valued obvious winner is rarely the best use of capital. The suppliers, infrastructure providers, enablers, and end users of a dominant trend often offer the same growth story at a far more attractive price.
Footnote:
1 Arnott, Rob, and Lillian Wu. "Membership Has Its Privileges: Who Pays the Premium?" Research Affiliates, CFA Institute Research Foundation Series, July 2026, researchaffiliates.com.
