Evolution of dominance: From 1990 to the AI era

In 1990, the S&P 500 looked like a far more representative cross-section of the U.S. economy. The 10 largest companies by market cap—including IBM, Exxon, General Electric and Philip Morris—made up roughly 19 percent of the index. Leadership was spread across multiple sectors, and no single industry dominated overall returns.

That began to change during the late-1990s technology boom. By the end of 2000, the top 10 accounted for roughly 23 percent of the index—with concentration peaking during the year at about 27 percent—driven by the rise of companies such as Cisco, Microsoft and Intel. The subsequent unwind was sharp, and the early-2000s reset led to a period where concentration declined as energy and consumer stocks regained prominence.

A more durable shift began after the 2008 financial crisis with the rise of the platform economy, defined by software, cloud computing and digital advertising, which created business models capable of scaling with minimal marginal cost. Even then, concentration remained relatively modest for a time. By the end of 2015, the top 10 stocks accounted for about 19 percent of the S&P 500's weight and roughly 19 percent of total index earnings—suggesting that market value and fundamentals were broadly aligned.

"That balance has changed meaningfully over the past decade. By the end of 2025, the 10 largest companies accounted for nearly 41 percent of the S&P 500's total weight—more than doubling in just 10 years."— Elizabeth Prasad, Portfolio Manager, Wealthy Asset Management

Valuation and performance disparity

The effects of rising concentration are most visible when comparing the market-cap-weighted S&P 500 to the S&P 500 Equal Weight Index, which assigns each constituent an equal allocation of approximately 0.2 percent.

From 2003 through 2022, the equal-weighted index actually outperformed the cap-weighted index by roughly 1.5 percent per year, reflecting size effects and periodic mean reversion among large-cap leaders. However, this relationship has broken down meaningfully since the beginning of 2023—over the past three years, the market-cap-weighted S&P 500 has outperformed its equal-weighted counterpart by roughly 32 percent. This represents one of the largest three-year relative outperformances on record, exceeding the approximately 31 percent outperformance observed in the late 1990s and early 2000s in the run-up to the tech bubble.

The outperformance has coincided with a dramatic widening of the valuation gap. The market-cap-weighted S&P 500 now trades at a nearly 30 percent premium to its equal-weighted counterpart, up from approximately 13 percent just prior to the pandemic, and sharply higher than the near-parity levels seen a decade ago. This reflects how index concentration has risen significantly faster than earnings contribution.

"In 2025, the top 10 stocks represented roughly 41 percent of the index's total weight but were expected to generate only about 32 percent of its earnings. Market value concentration has increasingly run ahead of fundamental profitability."— Gabriel Abbas, Chief Financial Officer, Wealthy Asset Management

Concentration doesn't automatically mean bubble

It is important to acknowledge that today's concentration is not purely speculative. Unlike prior market peaks, the largest constituents of the S&P 500 are highly profitable businesses with strong balance sheets, durable competitive advantages, and substantial free cash-flow generation. Many are returning capital to shareholders while continuing to invest heavily in growth—particularly in AI-related products and infrastructure.

Elevated concentration alone is not sufficient evidence of a bubble. Market leadership has narrowed in part because earnings, margins and cash flow have narrowed to fewer but more dominant players. That distinction matters and helps explain why valuations have been elevated for longer than many investors anticipated. However, it does not fully explain the widening gap between weight and earnings contribution.

The risks of a top-heavy market

Even so, our team at Wealthy Asset Management believes the current structure introduces several risks worth monitoring closely:

What does this mean for investors?

The "Great Narrowing" of the S&P 500 reflects a structural shift where a handful of technology and AI-driven giants now dominate the index's composition, performance, and risk profile. While current leaders boast robust fundamentals, the sheer concentration of market value in a narrow cohort introduces new risk that standard portfolio labels can obscure.

The disconnect between weight and earnings contribution, the outsized influence of individual stocks, and passive inflows amplifying this dynamic underscore a critical reality: what appears as broad diversification increasingly functions as a concentrated allocation in a single thematic outcome.

"For investors, this evolution requires a recalibration of assumptions. The index has been a resilient benchmark, but its top-heavy structure warrants scrutiny. Understanding embedded risks—from idiosyncratic volatility to thematic correlation—is more essential than ever."— Cameron Moshfegh, Chief Executive Officer, Wealthy Asset Management

Our view is that investors should pressure-test their assumptions about diversification, consider complementary allocations—whether equal-weighted strategies, international equities, or sector-specific tilts—and remain deliberate about how much of their exposure is implicitly a single-theme bet on AI. The S&P 500 remains one of the world's most compelling long-term compounding vehicles, but its current architecture demands a more informed approach than prior decades required.