A thematic ETF is built to isolate a trend, but the holdings often overlap with the S&P 500 more than a theme's name suggests. Across 380 funds, the typical thematic ETF holds 29% of its weight in stocks the S&P 500 already contains, and 74 of those 380 funds carry more than sixty percent index names. The gap between a theme and the broad market, in other words, is frequently a matter of degree rather than a clean separation.
That overlap varies widely by theme. Gold miners, silver miners, copper, cannabis, and coal show a median of 0% to 12% in S&P 500 names, with active share readings of 100% or close to it, meaning these portfolios look almost nothing like the index. At the other end, energy security and critical technology shows 86% in S&P 500 names with a 95% active share, and semiconductors, AI infrastructure and chips, digital infrastructure and data centers, and millennial and Gen Z consumers all show medians at or above 60%. For these themes, a large share of the portfolio is already index exposure, and the theme's distinct contribution is the smaller remainder.
Fees do not track this distinction in any simple way. Across 406 funds and 54 themes, the median fee is 0.60%, and when the fee is measured only against the part of each portfolio that differs from the S&P 500, the median fee for that difference is also 0.60%. Theme by theme, the two numbers are close in most cases, though a few move apart: broad innovation and megatrends shows 0.50% against 0.60%, AI infrastructure and chips shows 0.70% against 0.80%, and biotech shows 0.50% against a lower 0.30%. A fund's headline fee does not by itself say how much of that fee is being paid for the part of the portfolio that overlaps with an index a reader may already own elsewhere.
Overlap also shows up between funds within the same theme. Among 406 funds, 139 pairs hold portfolios that are more than half identical by weight. The list of pairs above sixty percent includes two electric vehicle and battery funds at 97%, two semiconductor funds at 86%, two gold miner funds at 84%, and three space funds paired at 81%, 79%, and 79% with each other. Fees within these pairs are not always matched: the semiconductor pair SOXQ and SOXX charges 0.19% and 0.33%, while the space pair ORBX and MARS charges 0.50% and 0.75%. Holding two funds this similar is close to holding one of them twice, at whatever combination of fees the two funds happen to carry.
One-year returns and index overlap moved in different directions depending on theme. Semiconductors, with a 66% median share in S&P 500 names, beat the S&P 500 in 9 of 9 funds and returned +98.9%. Defense, with a lower 39% median share, beat the S&P 500 in 0 of 14 funds despite its own distinct portfolio. The data does not show a fixed relationship between how much of a theme is already the index and how a fund performed over the period measured.
Readers can check any fund's current in-index weight, active share, and fee for the differing portion, along with its overlap against other named funds in its theme, on the live desk.