A thematic ETF is often sold as a way to target one idea, but many of these funds also carry a large slice of the same companies already sitting in the S&P 500. Across 388 funds, the typical thematic ETF holds 32% of its weight in stocks the S&P 500 also holds, and 72 of those 388 funds are more than sixty percent overlapping with the index by that measure. The flip side of overlap is active share, which measures how different a fund's weights are from the index; by theme, the medians run from 80% up to 100%, meaning even funds with meaningful index overlap still differ from the S&P 500 in most of their weight.
The theme-level detail shows how much this varies. Semiconductors has a median of 65% of weight already in the S&P 500 with active share of 85%, while Energy security and critical technology sits higher still, at 85% in-index weight. At the other end, Cannabis shows a median of 0% in the S&P 500 with active share of 100%, and Coal shows the same pattern. Gold miners, Silver miners, Rare earth and critical materials, and Lithium and battery metals all post single-digit medians for S&P 500 overlap alongside active share at or near 100%, consistent with these themes drawing on companies that sit outside the large-cap index.
Fees sit at a median of 0.60% across 415 funds and 54 themes. 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%, though this varies by theme: Semiconductors moves from a 0.30% median fee to 0.40% for the difference, and Artificial intelligence moves from 0.70% to 0.80%, while Cannabis moves from 0.80% down to 0.60%. One-year total returns ranged widely across themes, from +61.2% in Semiconductors to −32.0% in Blockchain and crypto equities, with "beat the S&P 500" counts showing, for example, 9 of 9 funds in Semiconductors ahead of the index over one year against 0 of 14 in Defense.
Separately from index overlap, some thematic funds overlap heavily with each other. The data identifies 126 pairs of funds across the full set whose portfolios are more than half identical by weight. The listed pairs include GII and IGF in Infrastructure at 97% weight overlap with fees of 0.40% and 0.37%, IBB and IBBQ in Biotech at 86% overlap with fees of 0.44% and 0.19%, and several Semiconductors pairs such as SOXQ and SOXX at 83% overlap with fees of 0.19% and 0.33%. Gold miners pairs AUAU/GDX, AUAU/GDMN, and GDMN/GDX each show overlap between 74% and 84%, with fees ranging from 0.35% to 0.51% among the three funds involved.
Readers can look up any individual fund's in-index weight, active share, fee, and one-year return, along with its listed overlap pairs, on ETFIQ's fund pages.