In fact, the study showed that 85 percent of employees subject to auto-enrollment participated in their DC plan, 18 percentage points higher than those that were not subject to automatic enrollment. However, most companies (85 percent of those offering automatic enrollment) only automatically enroll new hires, which is why the rise in participation has been so gradual.

"Employers are increasingly concerned that their workers are not adequately prepared to meet their future retirement savings needs," said Pamela Hess, director of retirement research at Aon Hewitt in a statement. "Automatically enrolling employees in company-sponsored DC plans is an easy way for companies to encourage workers to save more. However, this really is only a nudge in the right direction."

Interestingly, automation may actually hinder the amount of money contributed for those who are automatically enrolled in a plan. The analysis found that before tax contributions to DC plans were unchanged from 2009 at 7 percent of pay, but are still down slightly from pre-recession levels in 2007 (8 percent). Participants who were subject to automatic enrollment, however, contributed one percentage point less, on average, than their actively enrolled counterparts.

"Saving even just one percent less over a career has a dramatic impact on accumulation," Hess said. "Ultimately, it can lead to nearly a 15 percent loss in retirement income."

In addition to generally low contribution rates, many workers still aren't contributing enough to their 401(k) plan to receive matching employer contributions. Overall, 29 percent of plan participants contributed below the company match threshold, up slightly from 2009 28 percent.

The numbers are worse among participants who were auto-enrolled. Forty-one percent of participants who were automatically enrolled are not saving enough to receive the full match from their employers, compared to only 25 percent of participants who proactively enrolled.

"To drive higher savings rates, companies should consider combining automatic enrollment with automatic contribution escalation and target-date portfolios," Hess said. "Additionally, defaulting workers contribution levels at, or greater than the employer-match rate will also ensure greater success for employees struggling to save for retirement. Employers can also periodically back-sweep participants—or automatically enroll any worker who is not currently participating in the 401(k) plan—to ensure they are reaching all employees on an ongoing basis, rather than only at the point of hire."

Other key findings of the study include:

• Cumulatively, workers on average saved 10 percent of pay, including 4 percent from employer contributions.
• The average employee's total plan balance was $76,020 at the end of 2010, while the median balance was $24,680.
• The three largest asset class exposures (equally weighted) were premixed portfolios (33 percent), large U.S. equity (14 percent) and GIC/stable value funds (14 percent).
• The average worker's overall exposure to equities rose 0.5 percentage points in 2010, up from 66.9 percent in 2009.
• The median rate of return earned by employees in 2010 was 14 percent, down from 24 percent in 2009. The median, annualized, three-year rate of return earned (from 2008-2010) was just 2 percent, illustrating the dramatic impact losses in 2008 had on participant results.
• When available, 60 percent of workers invested at least partially in premixed portfolios, mainly driven by the popularity of target-date funds. Among those using premixed portfolios, just under half (46 percent) were fully invested in a single portfolio.
• Despite strong market returns in 2009, only 14 percent of employees made any sort of fund transfer in 2010, down from 16 percent in 2009.
• Nearly three in ten participants (28 percent) had a loan outstanding at the end of 2010, the highest in the ten years that Aon Hewitt has been tracking loans.
• In 2010, 6.9 percent of workers took a withdrawal from their DC plan, close to the record high of 7.1 percent in 2009. Among these, 20 percent were hardship withdrawals.