Exchange-traded funds will continue to challenge the entrenched dominance of traditional mutual funds, as investors increasingly flock to the low-cost, passively managed investment vehicles, says John Spence of MarketWatch.com.

Although ETFs' net assets are unlikely to take over mutual funds' assets, he says, they will increase market share and they have been growing at a faster pace. Assets in U.S.-listed ETFs exceeded $600 billion this year, its highest mark.

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