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METHODOLOGY · INDICATOR

Concentration

An index is only as strong as its weakest mega-cap.
WEIGHT
15%

WHAT WE MEASURE

Concentration measures how fragile the index is, specifically whether its performance is driven by a handful of mega-cap stocks rather than broad participation. When a small number of companies represent an outsize share of the index, a correction in just a few names can drag down the entire market, even if the majority of stocks are healthy.

WHY IT MATTERS FOR BUBBLE DETECTION

The rise of passive investing has created a self-reinforcing concentration dynamic: index funds automatically buy more of the largest stocks as they grow, pushing prices higher, which increases their index weight, which triggers more passive buying. This creates fragility. When the largest holdings sell off (often first in a crisis because they are most liquid), the entire index suffers regardless of the fundamentals of the remaining 490 stocks.
The primary inputs are: Top 10 Holdings Weight in the S&P 500, the Effective Number of Stocks (derived from the Herfindahl-Hirschman Index), and the divergence between the cap-weighted and equal-weighted S&P 500. Each is normalised and combined to produce the Concentration score.

KEY METRICS & THRESHOLDS

Top 10 Holdings Weight
The combined market capitalisation weight of the 10 largest S&P 500 constituents as a percentage of the total index. The historical average (1980–2010) was 18–22%. When this rises substantially above 28–30%, a shock to any single sector can become a systemic index event.
SOURCE: S&P Global; FactSet Index Analytics
Diversified
< 20%
Normal
20 – 25%
Concentrated
25 – 30%
High Risk
30 – 35%
Extreme
> 35%
Cap-Weight vs Equal-Weight Divergence
The performance gap between the cap-weighted S&P 500 (SPY) and the equal-weighted S&P 500 (RSP, Invesco). When the cap-weighted index significantly outperforms the equal-weighted version, it means a small number of mega-caps are carrying the market while the average stock lags: a classic sign of a narrow, fragile rally.
SOURCE: Invesco RSP vs SPY (Bloomberg)
Broad Rally
EW outperforming
Balanced
Gap < 3% (1-year)
Narrow
Gap 3 – 8%
Very Narrow
Gap 8 – 15%
Extreme Narrowness
Gap > 15%
Effective Number of Stocks (HHI)
Derived from the Herfindahl-Hirschman Index applied to index weights, this measures how many stocks are effectively driving index returns. The S&P 500 has 500 components, but if concentration is high, only 50–80 stocks may be "effective" drivers. The lower this number, the more fragile the index.
SOURCE: Herfindahl, O.C. (1950); adapted for equity indices by Research Affiliates
Well Distributed
> 200 effective stocks
Moderate
150 – 200
Concentrated
100 – 150
High Concentration
60 – 100
Extreme
< 60

HISTORICAL EPISODES

1972 – 1974
Nifty Fifty Bubble
SIGNAL
50 "growth" stocks at P/E 50–90x; dominated institutional portfolios
OUTCOME
Nifty Fifty stocks fell 70%+ on average by 1974
1999 – 2000
Dot-com Concentration
SIGNAL
Cisco, Intel, Microsoft dominated; Nasdaq top 10 = ~35% of index
OUTCOME
Nasdaq fell 78%; top stocks fell 80–99%
2007
Financial Sector Dominance
SIGNAL
Banks and financials = ~22% of S&P 500
OUTCOME
Financial sector fell 80%; S&P 500 fell 57%
2023 – 2024
Magnificent 7
SIGNAL
Apple, Microsoft, Nvidia, Alphabet, Amazon, Meta, Tesla = ~29–33% of S&P 500; drove ~60% of 2023 returns
OUTCOME
Ongoing; concentration at highest levels since 1970s

HOW TO INTERPRET THE SCORE

Diversified
0 – 25
Index returns are driven by broad participation. Resilient to single-sector shocks.
Moderate
25 – 50
Some narrowing in leadership. Monitor for further concentration.
Fragile
50 – 75
A small number of mega-caps are carrying the market. Significant sector-specific vulnerability.
Extreme Fragility
75 – 100
Historically rare concentration. A shock to the top holdings will cascade across the entire index.

ACADEMIC & INDUSTRY SOURCES

[1]
Siegel, J.: The Nifty Fifty Revisited1995
[2]
Herfindahl, O.C.: Concentration in the Steel Industry (HHI origin)1950
[3]
Arnott, R. (Research Affiliates): Reports on Concentration Risk in Passive Investing2020–2024
[4]
Goldman Sachs Equity Research: The Magnificent 7 and Market Concentration2023