This article is educational content explaining how thematic exchange-traded funds are generally constructed. It does not constitute investment advice and does not describe any specific current event, company, or security.
A fund can be labeled “clean energy,” “cybersecurity,” or “future mobility” and hold shares of a company that most investors would never associate with that theme at all. That is not an error. It is the predictable result of a construction process most shareholders never see: a chain of rules, thresholds, and committee decisions that starts long before the fund’s ticker appears on a screen. Understanding that process explains why two funds with nearly identical names can hold almost entirely different companies.
Defining the Theme and Screening the Universe
Every thematically screened exchange-traded fund begins not with a portfolio manager picking stocks, but with an index provider, or occasionally the fund sponsor itself, writing a rulebook. That rulebook has to translate an abstract idea, such as “artificial intelligence” or “water infrastructure,” into something a computer can test against company data. This typically means defining eligibility criteria: minimum market capitalization, minimum trading liquidity, listing on an approved exchange, and a revenue or business-description threshold tied to the theme.
The revenue threshold is where most of the apparent oddities originate. A provider might require that a company generate a stated percentage of its revenue, or classify a stated percentage of its business segments, under theme-relevant activity codes pulled from a data vendor’s classification system. Because many large, diversified companies operate several business lines simultaneously, a company with only a modest slice of theme-related revenue can still qualify if it clears the threshold, while a smaller, more focused company might be excluded for insufficient trading liquidity or a market capitalization below the stated floor. The screen is mechanical, not narrative: it does not ask whether a company “feels” like it belongs to the theme, only whether it satisfies the coded criteria.
Weighting the Survivors
Once the eligible universe is set, the index methodology assigns weights to each constituent. Thematic funds commonly use one of a few standard approaches: market-capitalization weighting, which favors larger companies; equal weighting, which gives every constituent the same starting allocation regardless of size; or a modified or “tilted” scheme that adjusts weights based on factors such as revenue purity to the theme, growth metrics, or liquidity. Some methodologies also apply capping rules, limiting any single holding or sector to a maximum percentage of the fund, to avoid excessive concentration in one company or industry cluster.
These weighting choices are disclosed in the fund’s prospectus and the underlying index’s published methodology document, both public records that any investor can review before or after buying shares. The choice of weighting scheme materially shapes how the fund behaves, since equal-weighted and cap-weighted versions of the same nominal theme can produce very different sector exposures and different sensitivity to the largest constituents.
Rebalancing and Reconstitution
A thematic index is not static. Providers typically distinguish between rebalancing, which resets constituent weights back toward their target allocations on a set schedule, often quarterly, and reconstitution, which re-runs the entire eligibility screen and can add or remove companies entirely. Reconstitution is usually less frequent, commonly annual or semiannual, because re-screening the whole universe and shifting holdings creates transaction costs and market impact that most methodologies aim to limit.
Between scheduled reconstitution dates, a company can drift out of alignment with the theme, get acquired, or see its business mix change, yet remain in the fund until the next review. This lag is a structural feature of rules-based indexing, not a flaw specific to any single fund, and it is one reason methodology documents typically include buffer rules or grace periods that prevent constituents from being added and removed repeatedly due to minor, temporary fluctuations near a threshold. Reading a fund’s methodology document, alongside its prospectus, remains the most direct way to understand exactly how, and how often, its holdings can change.
