Workers who had already been investing in their 401(k) for at least five years through 2008 saw their balances drop an average of 24% last year, compared with the S&P 500's sharp 37% decline, the Investment Company Institute and the Employee Benefit Research Institute said.

Because of their continued contributions over the past five years, their average account balance increased 7.2% a year. Among these consistent participants, the average account balance rose from $61,106 at the end of 2003 to $86,513 at the end of last year.

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