Fidelity Investments is the giant of ‘‘target date’’ mutual funds, one of the hottest ideas in 401(k) plans. For its customers, the payoff has hardly been outsized.

Target date funds are supposed to make investing hassle-free — and safer — by automatically shifting from stocks to bonds as investors near retirement. But research shows many Fidelity target date funds have routinely turned in worse returns in recent years than their biggest peers.