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Research Studies in Pension

Factor Affecting Retirement Mortality (FARM)

This FARM site consists of

• an Introduction
• an Abstract
• a Bibliography of research papers
• a collection of Summaries of the research papers.

Use the Table of Summaries to link to the summaries either by author or by risk factor.

Abstract | Bibliography | Introduction | Table of Summaries

Deceleration in the Age Pattern of Mortality at Older Ages

Horiuchi, Shiro, and John R. Wilmoth, 1998. Demography, 35(4): 391–412.

It is known that the rate of mortality increases almost exponentially with age through most of the adult age range but this tends to slow down at very old ages. The authors propose that the explanation for this deceleration is the selective survival of healthier individuals to older ages. This is the 'heterogeneity hypothesis' and the study explores this hypothesis by testing the following three predictions empirically:

• deceleration occurs for most major causes of death, both from disease and external causes
• deceleration starts at younger ages for more "selective" causes, e.g. those that are strongly determined by genetic and environmental risk factors, rather than accidents or some highly contagious infectious diseases.
• mortality deceleration shifts to older ages as the level of total adult mortality declines.

The paper provides mathematical illustrations of the prediction arguments within its Appendices.

The results appeared to strongly support the heterogeneity hypothesis, but there was the suggestion that factors other than heterogeneity may play a role in the changing mortality increases.

The study used mortality data for both males and females from Sweden for the period 1861–1990 and Japan for the period 1951–1990. Cohort mortality patterns were examined, as selective survival should operate within cohorts. Twelve causes of death were considered.

The age variations in mortality were examined using life–table aging rates (LARs), which measure the relative mortality increase with age. These were adjusted for changes in period mortality, to remove the effects of time trends in age–standardized death rates.

Overall, the data was compatible with the first prediction that the deceleration should be observed for most major causes. This pattern existed for both males and females. The two exceptions to the prediction were peptic ulcers and accidents not involving motor vehicles. Decline was found to begin at age 75 or 80, although it differed substantially among causes of death. Mortality deceleration started at relatively young ages for degenerative diseases, each of the major cancers and all cancers combined. Deceleration for infectious diseases (e.g. pneumonia and gastroenteritis) was at relatively old ages but there was no notable consistent mortality deceleration for external injuries under age 75. This satisfies the second prediction that deceleration occurs at younger ages for more "selective" causes, such as degenerative diseases. However, deceleration does not start early for two predominantly chronic disease categories: heart disease and peptic ulcer. The case for death from external injuries may be due to the random nature of accidents and that they are less of a function of aging processes. In general, in developed countries, degenerative diseases dominate total mortality.

The third prediction was that mortality deceleration should shift to older ages as the level of total adult mortality declines. Age 75 was chosen as the dividing point to distinguish younger old ages from older old ages. Upon first inspection, LAR trends for total mortality in Sweden and Japan do not seem highly compatible with the third prediction. A parametric model was required to illustrate the third prediction. This involved distinguishing two types of adult mortality; senescent mortality, which results from the age–related deterioration of physiological functions and therefore rises steeply with ages and background mortality, which is relatively independent of aging processes and therefore does not vary significantly with age. The effects of differential trends in background and senescent mortality were examined and it was found that mortality deceleration did tend to shift to older ages as the level of senescent mortality declined.

A limitation of the study was the tabulation of Japanese data by five–year age groups in a period format. This data was approximated into quasi–cohorts. There was the concern that the timing of deceleration may differ by cause of death and therefore affect the first prediction. The timing of deceleration was found to differ with cause of death, but this did not mask the first prediction, and indeed, correlated with the second prediction.

The individual–risk hypothesis is another possible explanation for why the deceleration of mortality increases at older ages. According to this theory, the age–related increase of mortality risk for individuals slows down at older ages due to physiological, evolutionary and/or reliability– theoretical reasons; i.e. it describes the aging processes in individuals. The paper did not consider this theory in detail. It acknowledged that although the results appeared to be broadly consistent with the heterogeneity hypothesis, it is possible that they are also compatible with specific versions of the individual–risk hypothesis.

The paper proposes that if the individual–risk hypothesis were to be tested in a comparable manner, there would be the need to specify the changes in aging processes that account for mortality deceleration and to state hypotheses that can be tested with available data.