A fee price war is underway. In a February filing, for example, the Vanguard Group said it was reducing fees on 10 funds with combined assets of $176 billion. Then SoFi, an online lender, said it planned to offer zero-fee E.T.F.s, while Charles Schwab and Fidelity increased the numbers of E.T.F.s. that could be traded without commissions. State Street Global Advisors, Defiance, BlackRock and DWS also announced that they were starting new low-fee funds, while JPMorgan Chase cut the fees on one of its United States stock funds to 0.02 percent.

By the end of 2018, XTF, a fund analysis firm, found that nearly three-quarters of all assets in E.T.F.s were held in funds with expense ratios of less than 0.2 percent — just $2 for every $1,000 invested.

Low-cost index mutual funds have been gaining popularity, too.

In September, Fidelity introduced two index mutual funds with zero expense ratios, even lower than Vanguard’s cheapest index fund, the 500 Index Fund Admiral Shares, which has a ratio of 0.04 percent. The Fidelity funds — the Zero Total Market Index Fund and Zero International Index Fund — took in $1 billion in their first month. In March, BlackRock announced that it would cut fees on its iShares S&P 500 Index Fund to $1.25 for every $10,000 invested from $4, making it BlackRock’s cheapest fund.

The failure of most active investment managers to match — much less beat — major indexes such as the Standard & Poor’s 500 or the Dow Jones Industrial Index has created a strong argument for cheap index funds that simply mimic those benchmarks.

“More investors are appreciating the virtues of index investing, and more investors are appreciating the value of low cost — the less you pay for your investments, the more you have in your account,” said Rich Powers, who heads E.T.F. management at Vanguard.