Population Projections by Prefecture

May,1997 published

I. Overview of Results of Projections

1. Total Population by Prefecture

(1) Total population will decline in 44 prefectures by 2020.

According to "Population Projection for Japan(median projection)" released before, the total population of Japan will peak in 2007 and then decline. According to the results of population censuses, the population had already decreased in 13 prefectures from 1990 to 1995. The result of the projection in Table 1 shows that the populations of more than half of the prefectures will decrease before the decline of national population(between 2000 and 2005). After that period, areas with decreasing populations will expand, and a population decline will be observed in 44 prefectures between 2020 and 2025. The populations in only three prefectures, Saitama, Shiga, and Okinawa will continue to increase at that point.
When examining the population in 2025 by an index in which the population in 1995 is set as 100, the indices of 33 prefectures will be below 100, indicating a decline of population from 1995. Also, the indices of 17 prefectures are below 90, which means the populations of those prefectures will decrease more than 10 %.
When analyzing population by regional block, the populations of Chugoku and Shikoku will decrease between 1995 and 2000. The population will decline in non-metropolitan areas first and then in metropolitan areas. After 2015, the populations of all regional blocks will start to decrease.

Table 1 Projected Total Population by Prefecture

(2) Population in prefectures surrounding Tokyo will Increase in the future.

The population census in 1995 indicated that Tokyo had the highest share the total population of Japan (9.4%), and followed by Osaka (7.0%). According to the present projection, these percentages will decrease to 7.8% and 6.0% in 2025 respectively. On the other hand, prefectures surrounding Tokyo, and Aichi, Fukuoka, and Miyagi, where the regional urban centers are located will increase their shares.

When analyzing population by regional block, the population of Southern Kanto showed the highest percentage, 25.9%, to the national population in 1995. The percentage of the population in this block will increase slowly and reach 26.8% in 2025. The percentage of the populations of Northern Kanto and Chubu will also grow, but the shares of the other regional blocks will decline.

2. Population by Age

(1) Youth Population will generally decline.

The national projection (median) estimates that the national youth population (aged between 0 and 14) will be decreasing by 2004 then increase by 2010 due to a sharp rise in the birth rate. According to the projection by prefecture, the youth population will decrease in all prefectures, reflecting such a tendency, but will increase between 2005 and 2010 in 18 prefectures. It will decline in more areas after 2010, and again in all prefectures in 2020.

Table 2 shows the percentage of the youth population to the total population of each prefecture. The percentages will increase in some areas from 2005 to 2010, but they will decrease in more areas afterward, and then in all prefectures after 2020. In 2025, the prefecture with the highest percentage of youth population will be Okinawa (17.1%) and the lowest figure will be that of Tokyo (10.6%).

Table 2 Percentage of Youth Population by Prefecture

(2) The Productive population will also generally decrease.

The national projection (median) indicates that the national productive population (aged between 15 and 64) will decline consistently in the future. According to the present projection, the number of prefectures with declining productive populations between 1995 and 2000 will be 34, and the productive populations of prefectures surrounding Tokyo will continuously increase. However, those populations will shift to a decline in all prefectures in 2015. Although they will recover in some prefectures, the trend of a decline persists in most prefectures.

The percentage of productive population to total population of prefectures will decrease in all prefectures by 2020. However, the national projection indicates an increase of the productive population percentage in 24 prefectures between 2020 and 2025, reflecting the growth of the national productive population between 2021 and 2028. The prefecture with the highest percentage of productive population will be Saitama (62.4%), and the lowest will be that of Shimane (53.7%).

(3) Aged population will increase in all prefectures by 2020.

The present projection indicates that the aged population (age over 65) will increase in all prefectures by 2020. The period with the highest growth rate of aged population is that from 1995 to 2000 in most prefectures. However, while showing some fluctuations, the growth rate of the aged population will be lower , and the aged population will decline in 31 prefectures between 2020 and 2025. Prefectures with a large aged population as of 2025 will be those in metropolitan areas, such as Tokyo, Kanagawa, Osaka, Saitama, and Aichi. Aged populations will double between 1995 and 2025 in Ibaraki, Saitama, Chiba, Kanagawa, Aichi, Nara, and Okinawa.

Figure Aged Population by Prefecture (1995, 2025)

(4) Percentage of aged population will exceed 30% in 14 prefectures in 2025.

The percentage of aged population to total population will consistently increase in all prefectures. According to the percentages by prefecture in Table 3, the number of prefectures where the figure exceeds 30% are four in 2015, eight in 2020, and 14 in 2025. The prefecture with the highest percentage of aged population in 2025 will be Akita (33.8%), and the lowest will be Shiga (22.8%).

Table 3 Percentage of Aged Population by Prefecture

II. Outline of Projection Method

1. Period of Projections

Population was projected every five years for 30 years from 1995 to 2025.

2. Method of Making Projections

The cohort component method was used as in the previous projection. By this method, the future population is projected applying four types of assumed figures such as fertility rate and net-migration rate to the population by sex and age in a certain year (initial population). The projections using Cohort component method require: 1) basic population, 2) future fertility rate, 3) future survivorship rate, 4) future net migration rate, and 5) future sex ratio at birth. In addition, after populations of prefectures were projected by the Cohort component method, figures by sex and age were adjusted uniformly so that the sum of prefectural figures would comply with the results of the national projection (median).

3. Initial Population

The initial population used as the benchmark of the projection was population (total population, i.e., including Japanese and foreigners) by prefecture, sex, age group (5-year age group) as of October 1, 1995 (1995 Population Census by Bureau of Statistics, Management and Coordination Agency). The population whose ages were "unknown" was proportionately distributed to every 5-year age group.

4. Assumptions on Birth Rate

Future fertility rate by age of mother is required for projecting the number of births in the future. To reflect the future trend of the fertility rate in national projections, the present projection calculated the relative disparities of fertility rate from the national figures (calculated by prefectures and by age of mother), and used them to determine the future fertility rates for each prefecture.. Because some age groups in some prefectures show idiosyncratic features in the trend of relative disparity, such geographical and age differences were taken into consideration when making the assumption on future figures.

When consistent trends of expansion/reduction in the relative disparities from the national fertility rates have been observed since 1980, such trends were reflected in the assumption. However, the geographical changes of differences in fertility, either expansion or reduction, have been slowing down and gradually stabilized for every age group. Therefore, we assumed that the change in the relative disparity from the period 1990-1995 to 1995 -2000 would be half of that change from 1985-1990 to 1990-1995. After the period 2000-2005, this relative disparity was fixed. On the other hand, when no specific tendency was observed in the relative disparity after 1980, the relative disparity of 1990-1995 was set to be constant in the future. Finally, the figures for future birth rate by prefecture and age of female were projected by applying the future relative disparity to the future fertility rate of the national projection(medium) (In Okinawa, however, the fertility rates of those older 35-39 years are still very high, and the future fertility rates at these ages will be estimated to be extraordinarily high if the relative disparity of fertility rate by age is fixed. For this reason, only in case of Okinawa, were the relative disparity of fertility rate in the age group older than 35-39 assumed to decrease exponentially to the period 2020 -2025).

5. Assumptions on Survivorship Rate

The survivorship rate means the probability with which population of age x survives for the next five years (to age x+5). The present projection assumes that the future survivorship rates by prefecture, sex, and age follow the trend of survival ratio in the national projections. At the same time, considering the recent reduction of regional disparity in survivorship rate, it was assumed that disparities among prefectures continue to be reduced in the future.

The life table of 1995 was created for each prefecture, and using this and the life table of 1990 (Statistics and Information Department, Minister's Secretariat, Ministry of Health and Welfare), the survivorship rates by prefecture, sex, and age group for 1990-1995 period were calculated. Then the relative disparity of survivorship rate from the national figures were calculated by sex and age group for each prefecture. These relative disparities were assumed to diminish linearly to 0 at 2090-2095 period. This target period is selected because when applying the regression line to the tendency of reduction in the inter-prefectural differences (standard deviation) of life expectancy at age 0, such difference become zero in 2084 for males and in 2099 for females.

When setting the future survivorship rate by prefecture, sex, and age group, the future relative disparities as assumed above were applied to the future survivorship rates in national projections (median).

6. Assumption of Net-migration Rates

The net migration rate means the percentages of net inward migrants to the population. Specific trends are difficult to find in the net migration rate by age group, because the economic situation of the nation or each prefecture in each period has large impacts on migration. On the other hand, according to the "Annual Report of Basic Resident Registers," the migration rate in Japan has continued to be somewhat stable, although at a low level . In addition, the net migration rate by regional block shows no significant changes after 1992. Based on these facts, it was assumed that the net migration rate in 1990-1995 period would be constant in the future for both males and females in each age group. Namely, the net migration rate between 1990 and 1995 was calculated for each sex and age group using cohort survivorship rate method, and this rate was assumed to be constant in the future.

7. Assumption for Sex Ratio at Birth

As in the national projections, the sex ratio was set at 100 females to 105.6 males based on national figures for the past five years.

8. Assumption on Figures in Hyogo and Osaka

Because the Great Hanshin Earthquake had a tremendous impact on the survivorship rate, fertility rate, net migration rate in Hyogo prefecture, the influence of the earthquake was removed from each rate, and the future figure was assumed based on such an estimated rate. The population that outwardly migrated from Hyogo might return to Hyogo as restoration proceeds, but it is difficult to estimate the degree of return at this time. For this reason, the present projection assumed that half of the outwardly migrated population caused by the earthquake would return to Hyogo from other prefectures after five years and one-fourth more after 10 years.

On the other hand, Osaka accepted more than half of the outwardly migrated population from Hyogo, thus net migration rates in Osaka between 1990 and 1995 were also influenced significantly by the earthquake as they were in Hyogo. Based on this fact, the impact of the earthquake was also removed from the net migration rates of Osaka, and the future figures were assumed based on those estimated rates.

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Population Projections by Prefecture