401(k) BALANCES GROW: Fidelity Investments says the average 401(k) account balance among its accountholders rose to $77,300 at the end of last year. That's up 12 percent from the average at the end of 2011, according to Fidelity, the nation's largest 401(k) administrator. In the final three months of 2012, the average account balance grew 2 percent.

RISING MARKET: The Standard & Poor's 500 index posted a 16 percent return last year, boosting investment returns in 401(k)s.

RISING CONTRIBUTIONS: The average account balance also rose because contributions from employees' paycheck deductions increased last year, as did matching contributions from employers.