CONCENTRATION RISKS: ARE BROAD MARKET ETFS TOO DEPENDENT ON MEGA-CAPS?

ROBIN PERSAUD

Co-Presenters: Anthony Kazandhizi, Leyla Adem

College: College of Business and Public Management

Major: BS.FINANCE

Faculty Research Mentor: Wang, Bo  

Abstract:

This research analyzes whether broad market, capitalization weighted ETF’s have become structurally concentrated in a small cohort of mega-cap equities, potentially undermining true diversification. As of 2025–2026, flagship ETF’s such as SPDR S&P 500 ETF Trust (SPY) and Vanguard S&P 500 ETF (VOO) exhibit historically elevated Top 10 concentration levels, with mega cap technology firms representing a disproportionate share of total index weight. Since cap weighting mechanically increases allocation to outperforming stocks, sustained outperformance by firms such as Apple Inc. and Microsoft Corporation has amplified index exposure to a narrow leadership group.Using Bloomberg data, this study quantifies concentration trends through Top 10 weight analysis, sector allocation shifts, and the Herfindahl Hirschman Index (HHI). A comparative framework evaluates cap weighted ETF’s against equal weight alternatives such as Invesco S&P 500 Equal Weight ETF (RSP), measuring differences in volatility, beta concentration, and drawdown sensitivity. Scenario stress tests model the impact of a 20–30% correction in the top five holdings, estimating the resulting decline in SPY and VOO relative to RSP under identical market conditions.Preliminary findings suggest that broad market ETF’s increasingly derive performance from a concentrated subset of mega cap constituents, reducing effective diversification despite large index breadth. This structural concentration raises systemic considerations, particularly given the scale of passive flows and index linked capital. While cap weighted ETF’s remain cost efficient and liquid vehicles, market participants should recognize that “broad exposure” may functionally represent concentrated exposure to mega cap technology leadership.This analysis evaluates whether diversification in modern passive portfolios is structurally resilient or increasingly dependent on a narrow segment of the equity market.