HOLT Portfolio & Quantitative Strategy "The Beta Deficit: Why Portfolios..."

Research evidence excerpt

HOLT Portfolio & Quantitative Strategy "The Beta Deficit: Why Portfolios..."

HOLT Portfolio & Quantitative Strategy UBS Research

A Market Built on Fewer Stocks

U.S. large-cap equity markets have reached elevated levels of concentration, with a

narrow cohort of mega-cap technology stocks now accounting for between 33% and

59% of total market across the common US large cap equity benchmarks (Figure 2). This

represents a structural departure from the past 40 years, with the current concentration

level roughly three times the long-run average. As a result, the benchmark is increasingly

defined by a small group of dominant stocks rather than a diversified representation of

corporate America. For active managers, this means that portfolio outcomes are

increasingly determined by exposure to, or divergence from, this concentrated group.

The extent of this concentration is further illustrated when concentration is measured

through the Herfindahl-Hirschman Index, which sums the squared weights of index

constituents. In a perfectly equal-weighted universe of 1,000 stocks, each weight is

0.1% and HHI = 0.001. As concentration rises, HHI climbs toward its theoretical

maximum of 1.0. The inverse of the HHI provides a measure of the effective number of

names in the portfolio, representing the number of equally weighted stocks that would

produce the same concentration as today’s cap-weighted portfolio. The U.S. large cap

universe has less than 50 effective stocks today as a result of the high concentration

levels (Figure 3).

Figure 2: Growing Representation of Mag 8 Figure 3: Rising Concentration of US Market

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Source: UBS HOLT, the BLOOMBERG PROFESSIONAL service as of 6/18/2026.