A thematic ETF is sold as a way to own a specific idea, but the holdings filings show many of these funds already carry a large slice of the S&P 500 inside them. Across 379 funds, the typical thematic ETF has 30% of its weight in stocks the S&P 500 already holds, and 74 of the 379 are more than sixty percent index names. That means for a meaningful share of this group, a large part of what an investor holds is not the theme at all, but the broad market wearing a theme's label.
The picture changes by theme. Energy security and critical technology shows a median 86% of its weight already in the S&P 500, the highest of any theme in the table, followed by AI infrastructure and chips and semiconductors, both in the low sixties. At the other end, cannabis and coal show 0% in the S&P 500, and silver miners and gold miners sit at 2% and 5%. Active share, which measures how different a fund's weights are from the index, tracks this in reverse: semiconductors has a median active share of 85%, the lowest in the table, while most other themes cluster in the 90s or hit 100%.
Fees do not fall once the index portion is stripped out. Across 54 themes and 405 funds, the median fee is 0.60%, and the median fee measured only against the part of each fund that differs from the S&P 500 is also 0.60%. Some themes move in either direction when fees are reweighted this way: artificial intelligence rises from 0.70% to 0.80% on this basis, while biotech falls from 0.50% to 0.30%. For an investor paying for a theme, this is the fee attached to the part of the portfolio that is actually distinct from a broad index holding.
A separate pattern shows up between funds rather than against the index. The second table lists 136 pairs of funds, out of all pairs checked, whose portfolios overlap by more than half their weight. The closest pair, PDBA and EVMT in agriculture, overlaps 98%. Three of the next four closest pairs sit in semiconductors and space, including SOXQ and SOXX at 82% and ORBX and MARS also at 82%. The summary describes holding both funds in a pair like this as close to holding one of them twice, since the bulk of the weight is shared rather than additive.
None of this says whether in-index weight, active share, or overlap is large or small for any particular purpose; it only reports what the filings show as of September 30, 2026. A reader can go to the live desk and check a specific fund's current in-index percentage, active share, and fee for the difference, or look up whether it appears in a high-overlap pair.