From the introduction:
As of September 30, 2015, state and local government retirement systems held assets of $3.56 trillion. These assets are held in trust and invested to pre-fund the cost of pension benefits. The investment return on these assets matters, as investment earnings account for a majority of public pension financing. A shortfall in long-term expected investment earnings must be made up by higher contributions or reduced benefits.
Funding a pension benefit requires the use of projections, known as actuarial assumptions, about future events. Actuarial assumptions fall into one of two broad categories: demographic and economic. Demographic assumptions are those pertaining to a pension plan’s membership, such as changes in the number of working and retired plan participants; when participants will retire, and how long they’ll live after they retire. Economic assumptions pertain to such factors as the rate of wage growth and the future expected investment return on the fund’s assets.
As with other actuarial assumptions, projecting public pension fund investment returns requires a focus on the long-term. This brief discusses how investment return assumptions are established and evaluated, and compares these assumptions with public funds’ actual investment experience.