Small-cap ETFs have recently fallen behind the S&P 500, though they outperformed the S&P 500 on August 26, ETF Trends reports. The shifting pattern has raised concerns among investors. The small-cap iShares Russell rose 2.4% and the SPDR S&P 500 ETF gained 1.5% during the day. The small-cap ETF declined 13.3% year-to-date through August 25, while the S&P 500 dropped 6.6%.

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