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As 401(k) plans have emerged as most people’s primary retirement savings account, the employers who sponsor these plans generally have beefed up investment choices and driven down fees, new research shows. Small plans remain the most inefficient by a wide margin.

The typical 401(k) plan has 25 investment options, up from 20 in 2006, and the average worker in a plan has annual plan costs equal to 0.53% of assets, down from 0.65% of assets in 2009, according to a study from BrightScope and the Investment Company Institute.

These findings suggest that after years of dumping traditional pensions and trying to avoid the role of retirement planner for workers, companies have on some level accepted their critical place in the retirement security equation. Change has come slowly. But the BrightScope/ICI study shows positive momentum in key areas.

Expense ratios are down by every measure: total plan cost, average participant cost, and average cost of invested dollars. Volumes of research show that costs are a key variable in long-term rates of return. That is why low-cost index funds, most often championed by Vanguard’s John Bogle, have become investor favorites and 401(k) plan staples. These funds account for a quarter of all 401(k) plan assets, the study shows.

Meanwhile, investment options have increased in a way that makes sense. The broadened choice is largely the result of adding target-date mutual funds, possibly the most innovative financial product for individuals in the past 20 years. These are one-stop investments that provide diversification and automatically shift to a more conservative asset allocation as you near retirement. Nearly 70% of plans now offer them, up from less than 30% in 2006, and in many plans they are the default option.

For those in small plans, though, the news isn’t so good. Expenses remain high: In plans with fewer than $1 million in assets, the average expense ratio for domestic equity mutual funds is 0.95%, versus 0.48% for plans with more than $1 billion in assets. Small plans are also far less likely to include an employer matching contribution: Just 75% of plans with fewer than $10 million in assets provide a match, vs. 97% of plans with more than $100 million in assets. Small plans are also less likely to automatically enroll new employees.

The most common match is 50 cents on the dollar up to 6% of annual pay, followed closely by a dollar-for-dollar match on up to 6% of pay.

One area with clear room for improvement is the default contribution rate in plans that automatically enroll new hires. Nearly 60% of these plans set the rate at just 3% of pay and 14% set it at 2% of pay. Only 12% had a default contribution rate of at least 5% of pay. Most advisers say you should contribute at least enough to get the full company match, which is often 6% of pay, and contribute even more if possible. Your savings goal, including the company match, should be 10% to 15% of pay.

The venerable 401(k) still has many problems as a primary retirement savings vehicle. Too many people don’t contribute enough, don’t diversify, and don’t repay loans from the plans; too many take early distributions and try to time the market. 401(k) plans don’t readily provide guaranteed retirement income, though that is changing, and because you don’t know how long you’ll live you have to err on the conservative side and save like crazy.

But we are headed the right direction, which is good, because for better or worse the 401(k) is how America saves.

Get answers to your 401(k) questions in the Ultimate Retirement Guide:
How Should I Invest My 401(k)?
Which Is Better for Me, Roth or Regular?
What If I Need My 401(k) Money Before I Retire?