Based on the Japanese Census in 1990, the proportion of population aged 65 and over was shown 12.5 percent. According to the median estimate in Population Projections of the Future Japanese Population issued in September 1992 by the Institute of Population Problems, the proportion of aged population will increase to 17.0 percent in the year 2000, and to 25.8 percent in the year 2025.
Such a trend toward the aging of the population also implies that the number of death will be also increased by the result of aging population. The total number of death in 1990 was recorded about 820,000 based on the Japanese Vital Statistics. According to the median estimate in Population Projections, the number of death will increase to 1,026,000 in the year 2000 and again to 1,694,000 in the year 2025.
In this article, we firstly examined the future levels of life expectancy with using the forecasting model of future life table modifying cause-age-sex mortality schedule. The result of simultaneous calculation showed that Japanese life expectancy will increase from 75.5 in the year 1990 to 79.5 in the year 2025 for male, and from 81.0 in the year 1990 to 86.5 in the year 2025 for female. Large part of future gain in life expectancy for male and female can be attributable to the decline of death rates from cerebrovascular disease and heart disease. The decline between 1990 and 1995 in mortality rate due to cerebrovascular disease contributed 38.3% of the total gain in life expectancy for male and 34.6 % for female, while the decline in mortality rate due to heart disease contributed 17.4 % of the gain in life expectancy for male and 22.6% for female. Moreover, death due to cerebrovascular disease accounted for 20.7% for male and 22.0% for female between 2020 and 2025, death due to heart disease accounted for 30.4% for male and 32.6% for female.
Secondly, we have tried to calculate the future number of death using various hypothetical populations. In particular, we have examined the relationship between the different future courses of mortality on the one hand and the future variation of cause-structure of mortality.
Different assumptions of mortality that concerned with time series level parameter for each causes of death will bring about quite different levels of aging population. For example, the model assumed constant mortality as in 1990 showed that the proportion of aged 65 and over would increase to 24.7 percent of total population by the year 2025. In contrast, the model assumed the future improvement of life expectancy that mentioned above showed that the proportion of elderly will increase to 27.5 percent. It should be noted that future improvement of mortality would be more important factor to the aging population.