Thematic ETFs are often sold as a way to get exposure that a broad market index does not already provide. The data suggests the separation is partial. Across 388 funds, the typical thematic ETF holds 32% of its weight in stocks already found in the S&P 500, and 72 of those funds have more than sixty percent of their weight in index names. Active share, a measure of how much a portfolio differs from the S&P 500 by weight, runs high across most themes, but the in-index share varies widely by theme.
Some themes sit close to the broad market. Energy security and critical technology shows a median 85% of weight already in the S&P 500. Semiconductors sits at 65%, AI infrastructure and chips at 63%, and digital infrastructure and data centers at 62%. Others sit far from it. Cannabis shows 0% in-index weight, coal 0%, silver miners 2%, gold miners 6%, and lithium and battery metals 8%. Defense and infrastructure both show a median 41% in the S&P 500 alongside active share of 98%, meaning the shared portion is concentrated in a smaller set of names while the rest of the portfolio is distinct.
Fees do not track this divide in a simple way. Across 415 funds and 54 themes, the median fee is 0.60%, and when the fee is measured only against the part of each fund that differs from the S&P 500, the median is also 0.60%. At the theme level the two figures are close for most categories: infrastructure, defense, gold miners, and biotech all show matching fee and fee-for-the-difference figures. A few themes show a wider gap, such as artificial intelligence at 0.70% against 0.80%, AI infrastructure and chips at 0.70% against 0.80%, and cannabis at 0.80% against 0.60% in the other direction.
Fund-level comparisons point to further overlap, this time between funds rather than between a fund and the S&P 500. The data identifies 127 pairs of funds whose portfolios are more than half identical by weight. The sampled pairs span gold miners, semiconductors, cybersecurity, infrastructure, biotech, water, and space. GII and IGF, both infrastructure funds, show 97% weight overlap with fees of 0.40% and 0.37%. IBB and IBBQ, both biotech funds, show 86% overlap with fees of 0.44% and 0.19%. Several semiconductor pairs, including SOXQ and SOXX at 83% and SHOC and SMH at 76%, also appear, as do three separate gold miner pairings built around AUAU, GDX, and GDMN.
Performance outcomes over the trailing year varied by theme and did not line up cleanly with in-index share. Semiconductors funds beat the S&P 500 in 9 of 9 cases with a median total return of +68.8%, while defense and infrastructure funds beat it in 0 of 14 cases each, returning −4.9% and +8.2% respectively. Biotech beat it in 8 of 8 cases, returning +39.0%, while clean energy and blockchain and crypto equities beat it in 0 of their fund counts, returning −2.8% and −31.9%.
Readers can look up any individual fund's in-index share, active share, fee, and one-year return, along with its theme classification, on ETFIQ's fund pages.