A thematic ETF is usually sold as a way to own a narrower slice of the market than a broad index fund. The data suggest that slice is often not as narrow as it sounds. Across 388 funds, the typical thematic ETF holds 32% of its weight in stocks that are already in the S&P 500, and 72 of those 388 funds have more than sixty percent of their weight in index names. The flip side of that number is "active share," the share of a portfolio that differs from the S&P 500: the median fund across 415 funds and 54 themes sits at a fee of 0.60%, and when that fee is measured only against the part of the portfolio that is not shared with the index, the median fee for the difference is also 0.60%.
The overlap varies widely by theme. Semiconductors, at 65% median weight already in the S&P 500, and AI infrastructure and chips, at 63%, sit at the high end, alongside artificial intelligence funds at 59% and energy security and critical technology at 85%. At the other end, gold miners funds carry a median of 6% in S&P 500 names, cannabis funds 0%, and lithium and battery metals funds 8%. Nuclear and uranium funds sit at 16%, space funds at 15%, and clean energy funds at 9%. These differences show up in active share too: gold miners, nuclear and uranium, and several metals-themed groups post active share at or near 100%, while semiconductors (85%) and metaverse and immersive tech (81%) run lower.
Fees do not track overlap in a simple way. Semiconductors, with the highest in-index weight among larger themes, carries a median fee of 0.30%, among the lowest in the table, and a median fee for the difference of 0.40%. Robotics and automation and space, both with lower S&P 500 weight, carry median fees of 0.80%. Cooling and thermal management, a single-fund theme, shows the highest fee in the table at 1.00%.
Overlap between individual funds, not just between a fund and the index, is also present. The data show 127 pairs of funds with portfolios more than half identical by weight. The listed pairs include GII and IGF in infrastructure at 97% weight overlap, IBB and IBBQ in biotech at 86%, and several semiconductor pairs such as SOXQ and SOXX at 83% and FTXL and SOXX at 78%. Fees within these pairs sometimes differ by a wide margin: IBB charges 0.44% against IBBQ's 0.19%, and SOXQ charges 0.19% against SOXX's 0.33%, despite the two holding largely the same stocks.
One-year returns varied across themes regardless of how much S&P 500 weight they carried. Semiconductors returned +67.6% with all 9 funds beating the S&P 500, while defense, at a similar 41% in-index weight to infrastructure, returned −2.0% with none of 14 funds beating it. Blockchain and crypto equities returned −35.0%.
Readers can look up any individual fund's in-index weight, active share, fee, and one-year return, along with its closest-overlapping peer funds, on ETFIQ's fund pages.