Indexy a trhy
Index Concentration: When a Few Companies Drive the Whole Market
Key takeaways
- Modern cap-weighted indexes are at their historically highest concentration since the 1930s.
- The top 5 S&P 500 companies now make up more than 25% of the entire index's value.
- High concentration means index results depend on the performance of a handful of companies.
- Concentration is not necessarily bad — large companies tend to be robust and profitable.
- Those wanting to reduce concentration can combine the S&P 500 with small-cap or equal-weight ETFs.
Index concentration describes how large a share of the total value is held by the biggest companies — and in recent years this concentration in the US market has reached historical highs.
How large is concentration today?
In the S&P 500, approximately five companies (tech giants such as Apple, Microsoft, or Nvidia) represent more than a quarter of the total market capitalization of the index. In other words: if these companies decline over a year, the entire index will feel their fall — regardless of the performance of the other 495 companies.
The situation is far from the ideal where 500 companies would contribute equally to returns. Weighting methods that can mitigate this are discussed in the article on index weighting.
Is concentration a problem?
It depends on the perspective. High concentration is not inherently a problem — these companies are large precisely because they are successful, profitable, and resilient. Historically, periods of high concentration have been associated with above-average returns.
The risk arises when the valuations of these companies decouple from their fundamentals. Then even a mild disappointment can cause a significant swing across the entire index.
Sector imbalance
High concentration goes hand in hand with sector imbalance. The technology sector makes up over 30% of the S&P 500. If you want more balanced exposure, consider:
- Adding a global all-world ETF — spreads capital beyond the US.
- Equal-weight variants of the index — every company gets the same weight.
- Small-cap ETFs — smaller companies have lower correlation with mega-cap names.
The passive investor and concentration
If you invest in a broad global ETF (All World), US concentration is notably lower than in a purely American product. How to build an initial portfolio with diversification in mind is covered in the portfolio construction guide. A comparison of All World and S&P 500 ETFs can be found in the dedicated analysis.
FAQ
What is index concentration?
It expresses how large a share of the index's value is held by the smallest group of the largest companies. The higher the number, the more the index's return depends on a handful of names.
Is high concentration in the S&P 500 risky?
Not by itself — large companies are large for good reason. Risk increases if those companies are overvalued or if one sector dominates more than a third of the index.
How can I reduce concentration in my portfolio?
Add a global ETF, small-cap, or equal-weight variant of the index. Each of these approaches spreads exposure beyond the largest American mega-cap names.