Those falling account balances can be attributed to two factors, the report says, the first of which is that the business mix is changing. In 2015, new plans converting to Vanguard had lower account balances.

The second factor is auto enrollment, which leads to more people saving, but also to more accounts with small balances — since auto enrollment tends to particularly boost participation among younger and lower-paid workers, who “tend to exhibit lower savings behaviors.” By the end of 2015, the report says, more than a third of participants joined their plan under automatic enrollment.

Target date funds are also on the rise; 90 percent of plan sponsors offered target date funds at year-end 2015, up 14 percent from year-end 2010. Nearly all Vanguard participants (98 percent) are in plans offering target date funds, and 69 percent of all participants use target date funds. Almost as many — 62 percent — of participants owning target date funds have their entire account invested in a single target date fund, while 40 percent of Vanguard participants are wholly invested in a single target date fund, either voluntarily or by default. And 99 percent of plans with auto enrollment default participants into a balanced investment strategy, with 97 percent choosing a target-date fund as the default.

The use of automatic enrollment grew by 50 percent since year-end 2010, and by the end of 2015, 41 percent of Vanguard plans adopted it; that’s up five points from 2014. Because larger plans were more likely to offer auto enrollment, 63 percent of new plan entrants in 2015 were enrolled via auto enrollment.

Slightly more than 60 percent of all contributing participants in 2015 were in plans with auto enrollment; while that feature was initially only applied to new hires, now it’s applied to eligible nonparticipants in half of Vanguard plans with the feature. Seven in 10 automatic enrollment plans have implemented automatic annual deferral rate increases.