There is a particular kind of financial confidence that comes from owning multiple funds. Three ISA funds, perhaps four — a Vanguard LifeStrategy 80%, an HSBC FTSE All World Index, a Fidelity Index World, and maybe an iShares Core MSCI World ETF for good measure. It feels methodical. It feels safe. For approximately 3.2 million UK retail investors, it is quietly the opposite.
These four funds, which between them account for billions of pounds of UK retail ISA money, hold materially identical top-ten positions. Apple, Microsoft, Nvidia, Amazon, Alphabet, Meta, Tesla, Broadcom, Eli Lilly, JPMorgan Chase. The weightings shift by fractions of a percentage point. The underlying reality does not shift at all. Spreading your ISA across multiple global index trackers does not reduce your exposure to US mega-cap technology — it multiplies it.
The Numbers Behind the Overlap
To understand the scale of the problem, consider the actual portfolio construction of the UK's most popular ISA funds as of early 2026.
The HSBC FTSE All World Index Fund tracks the FTSE All-World Index, which allocates approximately 64% of its weighting to North American equities. Its top ten holdings account for roughly 22% of the entire fund. The Vanguard LifeStrategy 80% Equity Fund holds Vanguard's own index funds as its building blocks, but those underlying funds — the Vanguard U.S. Equity Index and the Vanguard Global Stock Index — carry identical mega-cap US concentrations. The Fidelity Index World Fund tracks the MSCI World Index, which is approximately 73% US-weighted. The iShares Core MSCI World ETF (SWDA) tracks the same MSCI World benchmark.
An investor holding equal portions of these four products does not hold four diversified positions. They hold one highly concentrated bet on US large-cap technology, expressed four times over.
Using publicly available fund factsheets from each provider's website (HSBC Invest Direct, Vanguard UK, Fidelity, and iShares as of Q1 2026), a simple weighted overlap calculation reveals that an investor splitting their ISA equally across these four funds would have approximately 18–22% of their total portfolio sitting in just five stocks: Apple, Microsoft, Nvidia, Amazon, and Alphabet. That is before accounting for any direct shareholdings they might also hold.
Why This Matters More in 2026 Than It Did Five Years Ago
US equity concentration inside global index funds is not a new phenomenon, but the degree of that concentration has accelerated significantly. The MSCI World's US weighting stood at roughly 60% in 2019. It now sits above 73%. The passive investing revolution, which has been broadly positive for UK retail investors in terms of cost reduction, has had an unintended consequence: it has funnelled enormous sums of capital into the same narrow set of companies, inflating their index weightings, which in turn attracts more passive capital, which inflates their weightings further.
For UK investors, this creates a specific vulnerability. Sterling-denominated returns from these funds are exposed not only to the operational performance of US technology companies but also to GBP/USD currency movements. When sterling strengthens — as it did materially in the first quarter of 2026 — global index fund returns in pound terms are eroded even when underlying share prices are rising in dollar terms.
The concentration risk is further compounded by valuation. US technology mega-caps continue to trade at price-to-earnings multiples that are historically elevated. An investor who believes they hold a diversified global portfolio may be carrying far more valuation risk than they realise.
How to Run a Free Overlap Check on Your Own ISA
The good news is that diagnosing portfolio overlap requires no specialist tools and no financial adviser. The following process takes approximately twenty minutes.
Step one: List every fund you hold. Include all ISA funds, pension funds if relevant, and any ETFs held on platforms such as Hargreaves Lansdown, AJ Bell, Interactive Investor, Trading 212, or Freetrade.
Step two: Pull the top-ten holdings for each fund. Every fund is legally required to publish this information. For HSBC funds, visit the HSBC Global Asset Management factsheet library. For Vanguard funds, visit Vanguard UK's fund pages. For iShares ETFs, use BlackRock's product pages. For Fidelity funds, use Fidelity's fund factsheet portal.
Step three: Use a free overlap tool. The website ETFrc.com allows users to enter two ETF tickers and instantly see the percentage overlap between them. For UK-listed funds not covered by that tool, the Morningstar X-Ray tool (accessible via Hargreaves Lansdown's portfolio analysis feature for HL account holders, or directly via Morningstar UK) will aggregate holdings across multiple funds and show your effective single-stock concentrations.
Step four: Calculate your true US technology weighting. Add up the combined weighting of Apple, Microsoft, Nvidia, Amazon, and Alphabet across all your funds, weighted by how much of your portfolio is in each fund. If that number exceeds 15%, your portfolio is carrying meaningful concentration risk that the word 'diversification' does not justify.
What Genuine Diversification Actually Looks Like
True diversification in 2026 means exposure to asset classes and geographies that behave differently from US large-cap technology — not just different funds that hold the same underlying assets.
Several approaches are available to UK retail investors within an ISA wrapper:
Factor tilts: Funds such as the Vanguard Global Value Factor ETF (VVAL) or the iShares MSCI World Value Factor ETF (IWVL), both available on Hargreaves Lansdown, AJ Bell, and Interactive Investor, provide exposure to global equities screened for value characteristics. Value stocks have historically low correlation to growth-dominated mega-cap indices over medium-term horizons.
Emerging markets: The iShares Core MSCI Emerging Markets IMI ETF (EMIM) offers exposure to economies — India, Brazil, Taiwan, South Korea — that are structurally underrepresented in MSCI World and FTSE All-World indices. Available on all major UK platforms.
UK equities specifically: Despite years of relative underperformance, the FTSE 100 and FTSE 250 provide genuine geographic and sector diversification from US technology. The Vanguard FTSE UK All Share Index Unit Trust or the iShares Core FTSE 100 ETF (ISF) offer low-cost exposure. UK equities are significantly underweighted in most global index funds relative to the UK's share of global GDP.
Small-cap allocation: The Vanguard Global Small-Cap Index Fund provides exposure to thousands of smaller companies that do not appear in the top-ten holdings of any mainstream global tracker.
Alternatives: Gold ETCs such as the iShares Physical Gold ETC (SGLN) and fixed income via the iShares Core UK Gilts ETF (IGLT) provide genuine low-correlation ballast — both ISA-eligible and available across major UK platforms.
The ISA Deadline Consideration
With the 5 April 2026 ISA deadline now imminent, many investors are making final contributions for the 2025–26 tax year. Before directing that capital into a fourth global tracker, it is worth spending twenty minutes on the overlap check described above. The tax efficiency of the ISA wrapper is valuable precisely because it shelters genuine long-term growth — and that growth is harder to achieve when your 'diversified' portfolio is effectively a single concentrated position expressed through multiple fund names.
The Verdict
Holding multiple global index trackers is not diversification — it is the same bet, repeated, and the 3.2 million UK investors doing it deserve to know before the tax year closes.
This article is for informational purposes only and does not constitute financial advice. Your capital is at risk. Past performance is not a reliable indicator of future results.