Birth Trends, Survival Improvements, and Aging
Kenji OTANI

Utilizing equations based on the assumption t O, preceding studies tried to decompose a change in the proportion of aged persons or the average age between two times separated by 5 or 10 years. But this study showed that an interaction effect between birth and survival effects becomes sizable when t is not small enough. Decompositions of a change in the proportion of aged persons on the basis of the stable population theory emphasized the contribution of fertility change. This study confirmed that cohort survival improvements largely contributed to changes in the proportion of aged persons and the average age since the late 1970s. Particularly, the cohort survival effects for those belonging to the age group 65+ at the time concerned was great. But the survival improvements at the stage 65+ for male cohorts aged 65+ did not contribute to an increase for both the proportion of aged persons and the average age, while they contributed to a slight rise for women. This indicates that it is difficult to make the case that the mortality decline at 65 and over is the primary factor to facilitate aging in Japan. It is expected that the birth effect will rise again when the baby-boom cohort swells the aged population in the near future.

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