Defined benefit pension plans seem to have outperformed defined contribution retirement plans in most years since the end of the dot-com boom.

Towers Watson & Company, New York (NYSE:TW), makes a case for that conclusion in an analysis of Form 5500 plan data filed with the U.S. Department of Labor through 2008 and a separate analysis of results from 97 large plan sponsors that are on the firm’s list of the 100 biggest defined benefit plan sponsors.

When the firm looked at the results for 2008 and earlier, it focused on employers that sponsor just one defined benefit plan and one 401(k) plan, each with at least 100 participants.

Defined benefit plans in the sample generated higher investment returns than the defined contribution plans studied from 2001 to 2008, the firm found.

In 2008, the relatively conservative investment mix in defined benefit pension plans helped them hold their investment drop to about 23%, compared with a drop of about 26% for the defined contribution plans. Similarly, performance was about 2.25 percentage points better at defined benefit plans during the 2000-2002 slump than at defined contribution plans, the firm says.

In 2008, the difference between small defined benefit plans and small defined contribution plans was large: smaller defined benefit plans achieve returns that were 5 percentage points better than the returns achieved by defined contribution plans.

In 2009, as the stock market recovered, defined contribution plans generated an average positive return of about 19.1%, compared with an average positive return of about 18.5% at defined benefit pension plans.

“This marked the first time since the dot-com era and the 1995 – 2000 bull market that [defined contribution] plans outperformed [defined benefit] plans,” Towers Watson says.

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