ETF Return Skew
ETF return skew is the extension of Bessembinder's stock-market finding to funds: fund returns are so lopsided that a handful of funds create the outperformance while most lag the benchmark — and the narrower the fund, the worse the odds. A broad index ETF sidesteps it; a thematic ETF walks straight into it.
What it is
The finance professor Hendrik Bessembinder showed that individual-stock returns are extremely skewed: since 1926 most US stocks underperformed one-month Treasury bills, and the best-performing 4% of companies account for all of the market's net wealth creation. ETF return skew is the same distribution one level up — across funds instead of stocks. Because a fund's long-run result depends on whether it holds the rare mega-winners, the returns of funds are just as lopsided: a few win big, most lag the benchmark, and the gap widens the longer you hold.
The evidence
Bessembinder's Mutual Fund Performance at Long Horizons (US equity funds, 1991–2020) is the cleanest proof. Over a one-month horizon about 47% of funds beat a plain S&P 500 ETF — a coin flip. Over a thirty-year horizon only 5.5% did. More than two-thirds underperformed the index outright; over 20% failed to beat one-month Treasury bills; and fund investors lost roughly $1.02 trillion in aggregate versus simply holding the index. The thematic-fund record rhymes: on Morningstar's data only about 16% of global thematic funds survived and beat global equities over five to ten years, and the average thematic ETF lagged its broad benchmark by 8.5 percentage points.
Why it happens
Skew is a property of the market, so any fund that narrows the universe inherits it. A broad, cap-weighted index owns the whole distribution — the winners come free with everything else. The moment a fund restricts itself — to a theme, a sector, a country, a factor — it makes a double bet: that the slice is right, and that the next decade's winners live inside it. If they don't, the fund is mathematically guaranteed to miss them while keeping the long tail of losers. That is not diversification; it is a concentrated bet that feels diversified.
The exception that proves it
This is not “ETFs underperform.” A broad passive index ETF is designed to track the haystack, not beat it — and tracking the haystack is the winning move the skew argues for. In Bessembinder's study the S&P 500 ETF is the yardstick the trillion dollars was measured against, not the loser. The wealth destruction is specific to narrow, thematic and actively managed funds versus a broad benchmark.
How to use it
Own the broad index for the base of a portfolio, where the skew works for you, and treat every thematic or single-name tilt as a deliberate, sized decision that has to clear a high bar — because the base rate is against it. It is also the honest frame for any curated index or model portfolio: these are attempts to sit on the winners' side of the skew, and the skew is exactly what makes that hard. The full argument is in the heresy The Haystack and the Needle; the portfolio application is the barbell.