If one is the loneliest number, then there are some lonely retirement accounts out there: More than half of 401(k) participants use only one fund.

The proportion of 401(k) participants using a single target-date fund (TDF) in their accounts increased to 51 percent in April 2018, according to “How America Saves 2018,” a report from Valley Forge, Pa.-base Vanguard. 

Ten years ago, just 13 percent of retirement plan participants were using a single target-date fund, according to the report.

Vanguard attributes the rise of TDF use to an increase in plans using automatic enrollment. Plans with an automatic enrollment boast 92 percent participation rates, compared with just 57 percent for plans with voluntary-only enrollment.

Though 97 percent of 401(k) participants have access to TDFs, 10 percent still use “extreme” allocations like 100 percent or 0 percent to equities, according to Vanguard.

Still, the report argues that the adoption of TDFs has helped save more retirement plan investors from themselves. The proportion of participants with broadly diversified portfolios has increased to 75 percent, up from approximately 50 percent 10 years ago, and the proportion of participants with concentrated stock positions has fallen by half over the same timeframe.

“Target-date funds have revolutionized investing for millions of Americans, providing a ready-made, diversified portfolio for retirement savers,” said Martha King, managing director and head of the Vanguard Institutional Investor Group, in released comment. “Many participants lack the time, willingness, and expertise to build and manage their retirement portfolios, and TDFs offer a professionally-managed investment option at a very low cost.”

TDF investors trade infrequently, according to Vanguard. Only 2 percent of the investors holding TDFs in its analysis executed a trade during 2017.

Released in April, “How America Saves 2018” analyzes the retirement savings behavior of the 4.6 million participants in defined contribution plans using Vanguard’s recordkeeping services.