Few corners of the fund industry generate as much excitement — and as many warnings — as thematic ETFs. Built around a compelling story such as artificial intelligence, clean energy, robotics, or genomics, these funds promise exposure to the companies best placed to benefit from a long-term structural trend. They can be genuinely engaging to invest in, but their concentrated nature and often high past valuations make them worth understanding carefully before allocating meaningful sums to them.
What a thematic ETF actually is
Unlike a broad market tracker following an index such as the FTSE All-Share or MSCI World, a thematic ETF is built around a specific idea about the future — a technology, demographic shift, or societal trend expected to drive growth in a defined group of companies. Rather than tracking a market by size or geography, it tracks an index constructed specifically to capture companies considered exposed to that theme, such as an index of companies involved in artificial intelligence hardware and software, or one focused on renewable energy generation and storage.
How the underlying index is built
Thematic indices are typically constructed by an index provider using a defined methodology — screening companies by revenue exposure to the theme, business activity classifications, or a committee-based selection process. Because these are usually narrower and more subjectively defined than a broad market-cap index, different providers' thematic indices covering the "same" theme can hold quite different underlying companies.
Why thematic ETFs appeal to investors
A clear, understandable story
Many investors find it easier to feel conviction about a theme they understand and believe in — the growth of artificial intelligence, for example — than about an abstract market-capitalisation-weighted index.
Access to companies not otherwise emphasised
Broad market trackers weight companies by size, meaning smaller, more specialised businesses at the forefront of an emerging trend may receive only a tiny allocation. A thematic fund can provide more concentrated exposure to precisely these businesses.
Alignment with personal interests or values
Some investors are drawn to thematic funds — clean energy or healthcare innovation, for example — because the theme aligns with personal interests or values, alongside the financial objective.
The concentration risk behind thematic investing
Narrower diversification
A thematic ETF might hold anywhere from twenty to a hundred or so companies, often concentrated in a single sector or a small number of related sectors, and sometimes in a small number of countries. This is materially less diversified than a broad global index fund holding thousands of companies across many sectors and regions.
Valuation risk
Popular themes often attract significant investor attention and capital inflows precisely when a trend is already well recognised, which can coincide with higher valuations for the companies involved. Buying into a theme after a period of strong performance carries the risk of buying at valuations that have already priced in much of the anticipated growth.
Theme definition risk
Because there is no single agreed definition of many themes, the fund's actual holdings may not match an investor's mental picture of the theme. A "clean energy" fund, for example, might include diversified utility companies alongside pure-play renewable developers, resulting in a different risk and return profile than expected.
Trend longevity risk
Not every widely discussed theme translates into sustained investment outperformance. Some themes fade in popularity, some underlying business models fail to become as profitable as anticipated, and some themes prove to be shorter-lived than the multi-decade growth story often implied in marketing materials.
Comparing thematic and broad market ETFs
| Feature | Thematic ETF | Broad market ETF |
|---|---|---|
| Number of holdings | Often 20-150 | Often hundreds to thousands |
| Diversification | Concentrated in a theme, sector, or region | Spread across sectors, sizes, and often countries |
| Typical ongoing charge | Often higher, reflecting specialised index construction | Often very low for mainstream indices |
| Volatility | Can be significantly higher | Generally more moderate, reflecting broader spread of risk |
| Role in a portfolio | Often considered a satellite or smaller allocation | Often considered a core holding |
Questions worth asking before investing in a thematic ETF
- How is the underlying index constructed, and what criteria determine which companies are included?
- How many holdings does the fund have, and how concentrated is it in particular sectors, countries, or individual companies?
- What is the ongoing charges figure, and how does it compare with a broad market alternative?
- How long has the theme's index existed, and how has the theme performed relative to broader markets over that time, bearing in mind past performance does not predict future results?
- What proportion of an overall portfolio would this allocation represent, and how would a significant decline in this specific holding affect the wider plan?
A worked example: sizing a thematic allocation
Suppose a hypothetical investor holds a diversified portfolio of £100,000, largely in broad global and UK equity trackers, and wants to add exposure to a clean energy theme they find compelling. Rather than committing a large proportion of the portfolio, the investor might consider allocating, say, 5% (£5,000) as a "satellite" position alongside the diversified "core", reasoning that even a severe decline in this single theme — for example a hypothetical 40% fall — would reduce the overall portfolio by only around 2%, while a strong run for the theme would still meaningfully add to returns. This is a hypothetical illustration of a "core-satellite" approach some investors consider, not a recommendation of any specific allocation size, which depends entirely on individual circumstances and risk tolerance.
Where thematic ETFs might fit in a broader strategy
Many investors who use thematic ETFs do so as a smaller, deliberately sized "satellite" allocation alongside a diversified "core" of broad market index funds, rather than as the foundation of a portfolio. This approach allows for participation in a theme of interest while limiting the impact on overall portfolio outcomes if the theme underperforms broader markets. As with all fund selection, this is an educational framing of one approach some investors use, not a personal recommendation.
How thematic ETFs are marketed, and why it matters
Thematic funds are often launched by providers during periods of high public interest in a particular trend, sometimes near the peak of media attention and investor enthusiasm for that theme. This timing is not necessarily coincidental: fund providers naturally launch products they believe will attract investor interest and assets, which can mean new thematic launches cluster around moments when a theme's popularity, and sometimes its underlying valuations, are already elevated. This is not a criticism of any specific provider, simply a pattern worth being aware of when assessing the timing of an investment in a newly launched thematic fund with limited historical track record.
Assessing a fund with a short history
Many thematic ETFs have only a few years of trading history, which makes it difficult to assess how the fund might behave across a full market cycle, including periods of stress. Where the underlying index has a longer "back-tested" history published by the index provider prior to the fund's actual launch, it is worth remembering that back-tested performance is calculated with the benefit of hindsight and may not represent how a live, real-money fund would have actually performed.
Overlap with existing core holdings
An easily overlooked risk when adding a thematic ETF to an existing diversified portfolio is overlap — many of the largest, most well-known companies associated with a popular theme are often already significant holdings within a broad global or sector index fund, given their scale. Adding a thematic fund on top can therefore increase concentration in those same companies more than an investor may realise, rather than adding genuinely new, diversifying exposure. Reviewing a thematic fund's top holdings against the top holdings of existing core funds is a useful, practical check before investing.
Fund closures and consolidation in the thematic space
Because thematic funds are launched in response to investor demand for a specific trend, a proportion of them fail to attract sufficient assets over time, particularly if the theme's popularity fades or performance disappoints, and are eventually closed or merged into other funds by the provider. A fund closure typically returns the investor's money at the prevailing net asset value, which may be at a loss relative to the original investment, and can also trigger a taxable disposal for holdings outside an ISA or SIPP at a time not of the investor's choosing. This is a practical risk specific to newer, smaller, or less successful thematic funds, and checking a fund's total assets under management (its size) can give some indication of how established and viable it currently appears, though size alone does not guarantee a fund's future continuation.
A balanced view: thematic investing as one part of a wider plan
None of the risks described above mean that thematic investing is inherently unsuitable for every investor — many find genuine value in the ability to express a specific view or interest through a fund, and diversified core holdings can comfortably absorb a smaller, deliberately sized allocation to a theme that underperforms. The key discipline many experienced investors apply is treating thematic funds as a genuinely optional, clearly bounded addition to a plan that would still function perfectly well without them, rather than as a required or dominant part of the strategy.
Key takeaways
- Thematic ETFs track specialised indices built around a specific trend, offering more concentrated exposure than broad market funds.
- Their appeal often lies in a clear narrative and access to companies underweighted in broad market-cap indices.
- Concentration risk, valuation risk, theme definition differences between providers, and trend longevity are all worth understanding before investing.
- Ongoing charges for thematic ETFs are often higher than for broad market trackers.
- Some investors treat thematic funds as a smaller "satellite" allocation alongside a diversified "core" portfolio, rather than a primary holding.
- Reviewing the fund's actual holdings and index methodology helps confirm the fund matches the investor's understanding of the theme.