Millennials are going all-in when it comes to using exchange-traded funds in their investment portfolios.

In fact, almost everyone — 96 percent — who responded to a recent Charles Schwab survey said they considered the passive index-tracking securities “a necessary part” of their investment strategy, and 91 percent said ETFs are their “investment vehicle of choice.”

The move is coming at the expense of individual stocks, with more than half of millennials surveyed saying they dumped all their equity holdings for ETFs.

Passive investing, which involves using funds that track market indexes like the S&P 500, the Dow industrials and multiple other trackers and strategies, has surged in popularity over the past decade. Assets held by the industry have surged from $531 billion in 2008 to more than $3.4 trillion currently, according to Statista.

Growth has occurred across age groups but has been particularly strong among the youngest cohort, which grew up with an era in which indexing became vogue. Investors prefer the funds for their ease of trading and low costs compared to mutual funds.

“Within a decade, we’ve seen ETFs grow to the point where investors now see them as a foundational investment vehicle,” Heather Fischer, vice president of ETF and mutual fund platforms at Schwab, said in a statement. “While this sentiment is particularly pronounced among millennial investors, it is reflected strongly across generations and genders.”

Schwab conducted the survey from 1,500 of its clients, 55 percent of whom said they consider ETFs “the primary investment type” in their future portfolio. The percentage was highest among millennials, at 79 percent of respondents, and lowest among “matures,” at 14 percent.

In all, millennials made up 35 percent of respondents, which was the largest group, while males made up 64 percent compared to 36 percent for females. ETF popularity was fairly equal among genders — they comprised 33 percent of female portfolios compared to 34 percent for men, while 57 percent of women said they expect to increase allocations versus 54 percent for men.

The surge in ETF popularity also is coming at the expense of individual stocks.

Of the total surveyed, 32 percent said they replaced all their stocks with ETFs and 58 percent said they had replaced some; for millennials, the numbers were 56 percent and 64 percent, respectively, the most for either response.

Schwab has been conducting the study for eight years. The firm is the fifth-largest ETF provider, with $115.9 billion in assets, according to ETF.com.

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