Thematic ETFs: why most underperform — and how to hold one anyway

Thematic funds are products, and products launch when they sell. Issuers bring thematic ETFs to market at the moment of maximum public excitement — which is reliably close to the moment of maximum valuation. Research on thematic fund launches (Ben-David and co-authors' work on specialized ETFs is the canonical study) finds the underlying stocks systematically underperform in the years after launch: investors receive the theme's most expensive moment as their entry price.

The clean energy cycle is the textbook case. The largest clean energy UCITS ETF roughly tripled in 2020, attracted the bulk of its assets near the peak, then spent years down more than 60% while broad indices made new highs. The theme was real — renewable deployment kept accelerating — and the average invested euro still lost badly. Themes and returns are different things.

The deeper problem survives even perfect foresight. First, prices already contain the story: by the time a theme has an ETF, the growth is in the valuations, and returns depend on the future exceeding expensive expectations, not on the theme merely succeeding. Second, themes are hard to capture: index committees must define which companies are "AI stocks", and the resulting baskets are full of tangential mid-caps at high multiples while the theme's biggest beneficiaries are mega-caps you already own through your world index — where they earned their way in by winning.

History keeps demonstrating that transformative technologies and shareholder returns diverge. Airlines transformed the world and destroyed capital for a century; the dot-com era's infrastructure builders went bankrupt while the eventual winners were barely public in 2000. A broad index fund needs no view on any of this: whichever companies end up monetising a theme, cap weighting will hold them, automatically, without an entry fee at peak hype.

Prohibition fails; containment works. If a theme has genuine pull, give it a strict budget: 5–10% of the portfolio for all satellite positions combined, funded from new contributions rather than by selling the core. Prefer the cheapest, largest fund tracking the theme — thematic TERs of 0.4–0.6% compound painfully on top of the structural headwinds — and check fund size, since thematic funds dominate the closure statistics when narratives fade.

Then impose the discipline the category is designed to defeat: write down why you are buying and what would make you sell, never average down into a collapsing narrative, and let winners be trimmed back to the budget cap rather than growing into core positions. Treat the position honestly as entertainment with upside — the moment it feels like a retirement plan, re-read the clean energy chart.

The reassuring truth underneath: you own every theme already. MSCI World and FTSE All-World hold the leading AI, energy-transition, robotics and security companies at market weight, added and upweighted as they win. The core portfolio is not missing the future — it is the only instrument that captures it without requiring you to time it.