A theme fund is sold as a way to own a slice of the future that sits outside the everyday index. The data shows the overlap is often smaller than that pitch implies, but not always. Across 380 funds with a computed number, the typical thematic ETF holds 29% of its weight in stocks the S&P 500 already carries, and 75 of those 380 funds are more than sixty percent index names by weight.
The theme-by-theme table shows how wide the range is. Gold miners, silver miners, copper, cannabis and coal all show medians in the single digits or zero, meaning their stated median in the S&P 500 is 5% or less, so almost none of their weight sits inside the large-cap benchmark. At the other end, Energy security and critical technology shows a median of 86% in the S&P 500, and AI infrastructure and chips, Digital infrastructure and data centers, and Semiconductors each show medians in the 62% to 63% range. Millennial and Gen Z consumers, with a single fund, shows 66%. Active share, the companion number showing how different the holdings are by weight, runs the other way in these cases: Semiconductors' median active share is 85%, Artificial intelligence 80%, and Metaverse and immersive tech 77%, all lower than most other themes on the list, where active share often sits at 95% or above.
Fees sit at a median of 0.60% across 54 themes and 406 funds. When the fee is measured only against the part of each portfolio that differs from the S&P 500, the median for that difference is also 0.60%, meaning the extra cost per unit of distinct exposure has not moved once the shared index portion is set aside. Individual themes vary. Biotech shows a fee of 0.50% against the whole fund but 0.30% for the difference, while Semiconductors shows 0.30% against the whole fund but 0.40% for the difference, and Artificial intelligence, Broad innovation and megatrends, Electrification and the grid, Water, Digital infrastructure and data centers, AI infrastructure and chips, and Metaverse and immersive tech all show the fee for the difference running higher than the fee for the whole fund.
Beyond overlap with the index, some theme funds overlap heavily with each other. The summary counts 139 pairs across all themes that are more than half identical by weight, a scale at which holding both is close to holding one of them twice. The listed pairs include EVMT and PDBA in Electric vehicles and batteries at 97% overlap, SOXQ and SOXX in Semiconductors at 86%, and AUAU and GDX in Gold miners at 84%, with fees on each side of these pairs sometimes matching and sometimes differing, as with SOXQ at 0.19% against SOXX at 0.33%.
A reader working from the live desk can look up any single fund's in-index weight, active share, and fee for the difference, and check whether it appears in the overlapping-pairs list before treating it as a separate holding.