This abstract consists of a concise list of conclusions from the analysis in this paper.

A. Assessing Recent Official US Vital Rate Forecasts

• In retrospect, it appears that over the last fifty years, the Census and Social Security forecasters attached too much importance to the most recently observed levels of fertility and mortality.

• Recent Census Bureau projections of US fertility, based on race/ethnic disaggregation, appear to be too high. Recent Social Security fertility projections appear reasonable, although the range may be too narrow in light of international experience.

• Recent projections of life expectancy gains by both Census and Social Security appear to be substantially too low, in light of past US experience and international levels and trends in low mortality countries.

B. Uncertainty in Population Forecasts

• The standard method for dealing with uncertainty in demographic (and many other) forecasts is the use of high, medium and low scenarios. This approach is deeply flawed, because it is based on very strong and implausible assumptions about the correlation of forecast errors over time, and between fertility and mortality. The random scenario method is an improvement, but it retains some of the same flaws.

• Stochastic population forecasts based on time series models of vital rates (Lee-Tuljapurkar) appear to offer some important advantages, although long forecast horizons in demography far exceed the intended use of these models, and it is necessary to impose external constraints on the models in some cases to obtain plausible forecast behavior. One should not rely on mechanical time series forecasts in any case; they should be assessed in relation to external information.

• A parsimonious time series model for mortality appears to perform well within sample in applications in various countries, and suggests future life expectancy gains in the US at roughly twice the rate projected by Census and Social Security.

C. Results of Population Forecasts

• Middle forecasts by Census, Social Security, and Lee-Tuljapurkar (LT) agree closely on the timing and extent of increase in old age dependency ratios as the baby boom ages (although LT are somewhat higher due to lower mortality), but Census shows some amelioration after 2040, due to higher fertility. After 2040, LT forecasts continue to increase, doubling by 2070 to .45, while Social Security forecasts increase to .41. The Social Security range is three times as wide as that of Census, reflecting inherent flaws in the scenario method.

• Middle forecasts of the Total Dependency Ratio by Census and LT agree fairly closely , but are somewhat higher than Social Security (LT is .88 in 2070; Social Security is .83). The Social Security range is extremely narrow, reflecting inherent flaws in the scenario method, but the Census range is far too narrow as well.

• In Social Security forecasts, the correlation between errors in forecasting Youth Dependency Ratios and Old Age Dependency Ratios is close to –1.0. In Census forecasts, it is moderately positive. These correlations result from the bundling of assumptions in scenarios. LT forecasts show a correlation of -.6 to -.4, indicating partially offsetting variations in the proportions of children and elderly, as one would expect.

D. Stochastic fiscal projections:

• We analyze the performance of Social Security projections of cost rates since 1950, for forecast horizons of up to 35 years. Performance was generally very good, with no systematic bias, small average errors, and root mean squared errors smaller than the published high-low ranges. Projections done from the mid-70s to the mid ‘80s have under-projected costs by 12%, however.

• Middle LT forecasts suggest that government expenditures on the elderly will increase by over 150% in relation to GDP by 2070, while expenditures on children and age neutral expenditures will remain flat. Taxes rise from 24% now to 38% of GDP in the median forecast to 2070 (if debt/GDP is constrained), while the 95% probability range for taxes in 2070 goes from 25% to 53% of GDP.

• Increased costs of OASDI account for nearly 30% of the increase in expenditures on the elderly, but a larger share, 57%, is due to health costs in the median forecast. Fixing Social Security will not take care of the long term budget problem.

• Investing 90% of the Social Security reserve fund in equities yielding 7% (real) would fix the system according to a deterministic simulation, but in a stochastic forecast there is still a two thirds chance of exhaustion, with a median exhaustion date of 2044, and a negative median (but strongly positive mean) Fund balance in 2050.

• Raising the payroll tax rate by 2% immediately should nearly put the system in long term actuarial balance according to Social Security projections, but still leaves a 75% chance of fund exhaustion before 2070 in LT stochastic forecasts.

• Raising the normal retirement age to 71 by 2023 raises the median long term actuarial balance above 0 in LT stochastic forecasts, but still leaves a 43% chance of fund exhaustion before 2070.

Ronald Lee
Demography and Economics
University of California
2232 Piedmont Ave
Berkeley, CA 94720

Shripad Tuljapurkar
Mountain View Research
2251 Grant Road
Mountain View, CA 94024
email: tulja@mvr.org

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