Portfolio X-Ray · Research
Global ETF plus S&P 500: how much extra diversification are you buying?
Page updated · Rogha ETF
Published
A second ETF can add hundreds of holdings to your account without adding much new exposure. It can also leave more of your money riding on the companies you already own.
With major companies reporting earnings, it is a timely question. Microsoft’s next results are scheduled for 28 October. For someone holding several equity ETFs, the announcement can matter through more than one line in the account. Owning a company through three funds does not turn it into three different investments.
We compared fresh official holdings for three UCITS ETFs: a developed-world fund (A), an S&P 500 fund (B) and a Nasdaq-100 fund (C). All three holdings files are dated 23 October 2026, the latest disclosed files obtained for this research on 27 October. Fund names and ISINs are in the linked methodology.
One global fund, two possible additions
Our starting fund, A, follows developed-world large and mid-sized companies. It does not include emerging markets or the full small-company universe. “Global” here describes that particular scope, rather than every global ETF.
We then compare keeping that exposure alone with putting 80% in A and 20% in either the S&P 500 fund, B, or the Nasdaq-100 fund, C. These are illustrations of equity exposure, not recommended allocations. We remove cash and derivatives and scale each fund’s disclosed positive equity holdings to 100% before combining them.
| Illustrative equity portfolio | US weight | Ten largest companies | Information technology |
|---|---|---|---|
| A · Developed-world fund | 73.0% | 28.2% | 30.2% |
| B · S&P 500 fund | 100.0% | 39.5% | 37.9% |
| C · Nasdaq-100 fund | 97.1% | 49.5% | 57.4% |
| 80% A + 20% B | 78.4% | 30.5% | 31.8% |
| 80% A + 20% C | 77.8% | 32.3% | 35.7% |
Rogha calculations from issuer holdings dated 23 October 2026. Equity-only weights; country means issuer-assigned country of risk, not revenue location. Technology is the disclosed sector category, not a measure of all AI-related businesses.
Most of the addition is already there
Of B’s equity weight, 98.8% is in securities already held by A. For C, the figure is 98.4%. These figures answer a specific question: how much of the added fund goes into investments that appear in the starting fund?
They do not mean the funds have the same weights. The S&P 500 portfolio puts more of its money into large US companies. Nasdaq-100 applies a different selection rule and puts still more weight into several familiar names.
The combined portfolios contain some extra securities. But they account for only about 0.23% of the S&P 500 combination and 0.33% of the Nasdaq-100 combination. After our explicit company consolidation, newly represented companies account for about 0.23% and 0.20% respectively.
More funds, more weight in the largest companies
In this sample, the main effect is to change the amounts invested in existing companies. The ten largest account for 28.2% of the developed-world equity portfolio. They rise to 30.5% with the S&P 500 addition and 32.3% with Nasdaq-100.
A US tilt can be deliberate
Adding the S&P 500 fund raises US weight from 73.0% to 78.4%. The Nasdaq-100 combination reaches 77.8%. Nasdaq-100 is not exclusively a US-company index, and it is not a pure technology index. In this comparison it increases the technology sector weight more sharply than the S&P 500 addition.
There is nothing inherently inconsistent about wanting that tilt. An investor might deliberately favour the US market or a particular group of companies. But that is a different decision from seeking new sources of diversification. Adding another fund name does not settle which decision you have made.
What overlap does not tell you
Shared holdings show common exposure, not the full relationship between returns. Two companies can face similar risks even when their securities never overlap. Equally, a concentrated portfolio can outperform a broader one. This study does not test future returns, correlations or drawdowns.
Nor does the result mean every global fund has these weights. A developed-world tracker, an all-country fund and an equal-weight strategy can begin from different portfolios. Refreshing the underlying holdings matters more than carrying a percentage from one product into another.
During earnings season, a useful question is how much of your portfolio depends on the same businesses delivering. In these examples, the second ETF adds very little new company exposure. Its larger effect is to increase the weight of positions already in the account.
Explore further
Read our Learn guides on diversification, global ETFs and what an ETF tracks. Use Explore ETFs to research fund characteristics.
Related research: World, All-World and All Country benchmarks and active or passive ETFs.
What do you think?
We welcome comments, questions and corrections. Email hello@roghaetf.ie.
Read the methodology, fund identities and official sources.
Continue your research
- Understand ETF overlap
- Check sources of diversification
- Research global ETFs
- Research US ETFs
- Check availability before buying
Explanation and onward links updated 28 October 2026. Original 27 October research and 23 October holdings dates are retained. No holdings datasets or new calculated outputs were added.
S&P Dow Jones Indices: S&P 500 scope
Comments or corrections? Email hello@roghaetf.ie with the page and supporting source.