A fund labelled thematic does not always look different from a plain index fund once you open it up. As of September 28, 2026, the typical thematic ETF in this dataset holds 30% of its weight in stocks that the S&P 500 already owns, based on the median across 379 funds. That is the whole fund's overlap with the S&P 500, not a single stock's. At the far end, 74 of the 379 funds have more than sixty percent of their weight in names the S&P 500 already holds, meaning the theme sits mostly on top of a large-cap index rather than apart from it.
The overlap varies widely by theme. Semiconductors funds have a median 62% of their weight already in the S&P 500, and AI infrastructure and chips funds sit at 63%. Energy security and critical technology is higher still, at 86%. At the other end, gold miners funds show a median of just 5% overlap, silver miners 2%, and cannabis funds 0%, meaning almost none of their holdings are already in the S&P 500. Semiconductors, at 85% median active share, and AI infrastructure and chips, at 86%, are among the lower active-share readings in the table, meaning their portfolios differ from the S&P 500 by a smaller share of weight than themes like gold miners, silver miners or cannabis, all at 100% active share.
Fees do not track this pattern in a simple way. Across 405 funds spanning 54 themes, the median fee is 0.60%. When the fee is measured only against the part of each portfolio that differs from the S&P 500, stripping out the overlapping weight, the median fee for that difference is also 0.60%. Some themes shift when measured this way. Artificial intelligence funds carry a 0.70% median fee but a 0.80% median fee for the difference, since part of their cost is applied to holdings already available through an S&P 500 fund. Biotech runs the other way, with a 0.50% median fee falling to 0.30% for the difference.
Beyond overlap with the index, some thematic funds overlap heavily with each other. The data identifies 136 pairs of funds whose portfolios are more than half identical by weight, meaning holding both is close to holding one of them twice. The closest examples include PDBA and EVMT in agriculture at 98% weight overlap, AUAU and GDX in gold miners at 83%, and SOXQ and SOXX in semiconductors at 82%, each carrying its own separate fee.
Return patterns also differed sharply by theme over the one-year period shown. Semiconductors funds beat the S&P 500 in 9 of 9 cases with a median total return of +98.4%, while defense funds beat it in 0 of 14 cases with a median return of −0.3%.
Readers can check any fund's current in-index weight, active share, fee and pairwise overlap numbers on the live desk.