This paper aims at analyzing the relationships between net-migration and the transition of population distribution from the perspective of cohort analysis. The article consists of three parts: 1) review of the previous studies, through which the insufficient coordination of the two major theories on population concentration to the Tokyo Region is pointed out. 2) evaluation of the transition of population distribution in Japan, in which Gini's coefficient and the rate of concentration to the Tokyo Region by cohort illustrate some quantitative relationships between the two theories at issue. 3) presentation of a new projection method, where a basic concept of a new method for regional population projection and the preliminary trial for the Tokyo Region are discussed.
Firstly, the review of the previous studies on population concentration to the metropolitan regions came up with the following four points. D The various studies almost commonly recognized that a migration trend in Japan reached a period of transition in around 1970. 2) The major theories explaining the population concentration are summarized into economic-component and cohort-component theories, the latter of which appeared as a complementary viewpoint in the 1970s when the former theory relatively lowered its explaining power. 3) Cohort component theory maintains a close association with "potential outmigrants" hypothesis, which indicates that the decrease of potential migrant population was caused by fertility transition. 4) Quantitative analysis on relationships between the above two theories are, despite various attempts, yet to develop.
To establish a framework to integrate the two perspectives, the actual change of population distribution was firstly grasped. The comparison of Gini's coefficient by age in 1985 and in 1990 indicates that each cohort born before mid-50s demonstrated little temporal change in its regional distribution, while the distribution of four 5-year cohorts born in the 1940s and 1950s showed considerable inter-group variations. To comprehend the situations more concretely, a long-term time-series analysis on cohorts in the Tokyo Region was undertaken to examine the transition of each cohort's share to the national total. It is clarified that the above inter-group differentiation was attributed to the decrease of each cohort's share in the Tokyo Region at the time when these cohort members turned from the ages of 20-24 to 25-29. In other words, such variations hinged upon the intensity of each group's U-turn flow from the Tokyo Region, and the decline of each cohort's share in the Tokyo Region was considered to be affected by the employment conditions of the periods concerned.
Against this background, factors regulating net migration were proposed. Based on the above analysis three main factors such as "cohort scale" , "cohort-share change" , and "temporal share change" were abstracted. "Cohort-scale" factor demonstrated its largest contribution in the period of accelerated population concentration (from the later half of the 1950s to the first half of the 1960s) and of stagnant concentration (especially from the latter half of the 1960s to the first half of the 1970s). This factor explained 45% and 3596 of the total change in the above two decades respectively On the other hand the influence of "cohort share change" factor was highest in the 1970s, when the pace of concentration was slowing down. Its percentage of contribution was 20%. "Temporal share change" factor, or economic factor in other words, constantly played the predominant role and explained 45-100% of the total change throughout the periods examined.
This cohort-share analysis is applicable to the population projections at the regional block scale (which divides Japan into approximately 10 units). Although this method, named "cohort share extension method", needs further elaboration, it contains a methodological advantage that more realistic projections become possible through the incorporation of the various scenarios of economic change. A tentative estimation undertaken here was based on the scenario that the employment circumstances of the younger generations would continue to be tight. It ensued that the total population in the Tokyo Region would hit the peak at under 35 million in 2015, and that the population in 2025 was to be smaller by 6 million people than the figure estimated by cohort component method (utilizing net migration rates between 1985-1990). It should thus be noted that the prospective images of the Tokyo Region vary in accordance with the methods adopted for projection.