THE STUDY: A study by human resources consultant Aon Hewitt and investment adviser Financial Engines shows that workers who received some form of help with their 401(k) plans experienced annual returns on average of 3 percent better than workers who handled their own accounts.

DEFINING HELP: Workers who used target-date mutual funds, professionally managed accounts or accessed online advice were all deemed to have used help for purposes of this study. Their behavior from 2006 through 2010, and how it affected account risks and returns, was studied.

THE MISTAKES: Common investing mistakes helped account for the difference in performance. They include: pulling money out of stocks when the market tumbles, and then failing to reinvest before the market recovers; maintaining too much or too little risk; investing too much in the employer's company stock; failing to rebalance periodically, or being overly complacent about managing the account.