(Bloomberg) -- For a decade, a new kind of mutual fund has been taking over Americans' retirement portfolios. There's now evidence that target-date funds may be working. They're giving investors solid returns, data from research firm Morningstar show. Just as importantly, they're boosting those returns by protecting investors from their worst instincts.

That's a good thing, because the retirements of millions of Americans, and especially young people, now rely on target-date funds.

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