Over the past few decades, fewer and fewer workers have been covered by traditional employer provided "defined benefit" pension plans.
Instead, our income tax system has provided incentives for a growing and complex array of tax-advantaged "defined contribution" pension plans: 401(k) plans, 403(b) plans, 457 plans, traditional and Roth IRA plans, Keogh, SIMPLE and SEP plans, and more.
Here are some good papers to read about the economic implications of these developments:
"The Changing Landscape of Pensions in the United States", James M. Poterba, Steven Venti, and David A. Wise, December 2007
"Choice, Behavior, and Retirement Saving", Steven Venti, December 2004