Despite the arrival of a severe bear market in the period, the average account balance among American workers who consistently held 401(k) accounts from 1999 through 2005 increased 50%.

That was one of the findings of a new study of 401(k) account holders by the Employee Benefits Research Institute and the Investment Company Institute, both in Washington.

For workers studied, the average account balance increased 10% last year. The average account balance rose to $102,014 at year-end 2005, from $67,785 at year-end 1999, among participants who maintained accounts for the entire period.

The EBRI-ICI database contains a snapshot of account information for almost 18 million 401(k) plan participants–about 37% of the estimated 47 million U.S. workers who took part in 401(k) plans at year-end 2005, EBRI says.

The average 401(k) balance of all participants at year-end 2005 was $58,328, while the median balance (half of all accounts held more, and half less) was $19,398.

EBRI cautions, however, such year-to-year snapshots can be misleading, since the sample of 401(k) participants changes as older, high-value account workers leave the 401(k) system and younger, low-value account workers enter.

Among the study’s other findings:

o Company stock as a percentage of 401(k) plan assets continued to fall in 2005.

o Lifestyle and lifecycle mutual funds continue to gain popularity, especially among newly hired workers. Recently hired 401(k) participants in their 20s hold a larger share of their 401(k) accounts in balanced funds, including lifestyle and lifecycle funds, than did their peers in 1998.

o On average, 68% of participants’ assets were invested in equities at year-end 2005 through equity funds, the equity portion of balanced funds, and company stock.

o Only 10% of 401(k) participants approaching retirement age have an outstanding loan from their plan.

“The data demonstrate the power of persistence and the impact it has on an individual’s ability to accumulate sizeable gains in a 401(k) account,” commented ICI senior economist Sarah Holden, a coauthor of the study.