Article originally published by CNBC.

Written by Greg Iacurci

Target-date funds have ballooned in popularity over the past 15 years — yet many investors aren’t using them the way they were intended.

The funds were designed as a one-stop shop that put retirement savings on autopilot. Investors are meant to park their nest egg in one fund, generally based on their retirement year, which automatically shifts from stocks to bonds over time.

However, a third of investors aren’t limiting themselves to one target-date fund, according to 401(k) data from Vanguard. They’re piling other funds on top.

Click here to read the rest of this article on CNBC.com.