Literature review of long-term mortality projections.

Abstract

Overview Predicting life expectancy 75 years into the future today is equivalent to having a forecaster in 1929 predict life expectancy for 2004. The most accurate prediction about such a forecast is probably that it will be wrong. Each forecast in this review involves assumptions about the extent to which the past will represent the future and the appropriate definition of the relevant past. Most demographers who produce forecasts prefer to fit a trend line to historical data, treating fluctuations in gains in life expectancy as random fluctuations around an upwardly trending mean. Most actuaries prefer to adjust past trends based on their interpretations of the causes of these past trends and the likelihood of these past events being repeated in the future. In addition, some experts in the field of aging believe that the pace of mortality decline will slow down because of the biological limits of the human body; others believe that science can overcome current limits to life expectancy. Some experts believe that changes in lifestyle and the adoption of healthier behaviors could increase life expectancy significantly, even without further medical breakthroughs. This literature review compares the 2004 forecasts of the Board of Trustees of the Old-Age and Survivors Insurance and Disability Insurance (Social Security) Trust Funds with the recommendations of the 1999 and 2003 Technical Panels on Assumptions and Methods, both of whom use age-specific extrapolation techniques to recommend faster mortality improvement. The review then examines the recommendations of researchers who use life-expectancy extrapolation techniques to forecast convergence in life expectancies among developed nations. These researchers recommend even faster mortality improvements for the United States than do the two most recent technical panels. Finally, the review provides an overview of arguments in favor of a forecast that incorporates a theoretical maximum achievable life expectancy. These arguments generally imply a slowdown in the rate of mortality decline as the theoretical maximum life expectancy is reached. Studies by these experts are not directly comparable, but they are relevant to the extent that they contradict the assumptions of demographers, actuaries, and other forecasters who do not force their models to project only up to a theoretical maximum life span. Because the views of biomedical researchers who predict extremely large mortality declines are more speculative in that their predictions rely on large future medical breakthroughs to be achieved, they are not examined in this review. A determination of where the Social Security Trustees' mortality forecast lies within the range of opinions about future gains in life expectancy largely depends on which opinions are included in the range. Philosophically, those who predict the least amount of future gains in mortality reduction are probably those who believe that the biological limits of the human body (or senescence) will cause average life expectancy to eventually run into a maximum limit (Fries [1980] 2002, Olshansky and Carnes 2001). Some official forecasters in countries belonging to the Organization for Economic Cooperation and Development (OECD) are also on this end of the spectrum. Other official forecasters in the OECD believe that the speed of mortality decline will not be quite as rapid as in the past, but that there will still be a nonzero decline throughout the forecast period. This group includes the 2004 Social Security Trustees. Demographers who extrapolate age-specific rates in mortality decline tend to believe that the speed of mortality decline will not slow down, relative to past performance (Lee and Carter 1992; Wilmoth 1998; Tuljapurkar, Li, and Boe 2000). (There are some exceptions, however; for example, the 1999 and 2003 Technical Panels slowed mortality decline at younger ages more than has occurred in the past. The 2003 Panel also set the ultimate rate of mortality decline to zero in 2200. …

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