This study aimed to understand the age‐specific internal migration patterns in Japan where more than a quarter of the population is aged ≥65 years and the national population is on the decline, focusing on the urban hierarchy. Demographic efficiency, which is the ratio of net to gross migration, was calculated for each migration flow between the levels of urban hierarchy in Japan based on the 2020 census. The results showed that the overall upward migration, which is a movement from lower to higher levels in the urban hierarchy, was dominant. A variation in migration patterns by age confirmed that young adults, especially those aged 20–24 years, moved up the urban hierarchy unidirectionally, while people of preretirement and retirement age moved down. The older adults (aged 75 and over) were inclined to move up. Regarding sex‐based differences, the demographic efficiency of women was higher than that of men. Additionally, the upward movement of women was predominant in almost all age groups. These findings suggest the possibility that aggregated migration behaviours may vary by sex, which has received little attention in previous migration studies focusing on urban hierarchies in other countries. Furthermore, this study discussed policy implications based on the observed patterns, such as the impacts of older adults' migration on social security at each hierarchy level and a possible relationship between women's upward migration patterns and future fertility decline.

中文翻译：

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在城市层级中上下移动：基于 2020 年人口普查的日本特定年龄的内部迁移模式

本研究旨在了解日本特定年龄的内部移民模式，日本超过四分之一的人口年龄≥65岁，全国人口呈下降趋势，重点关注城市等级制度。根据 2020 年人口普查，计算了日本城市层级之间每次移民流动的人口效率，即净移民与总移民之比。结果显示，整体向上迁移（即城市层级中从较低层向较高层的流动）占主导地位。不同年龄的迁移模式的变化证实，年轻人，尤其是20-24岁的年轻人，单向在城市层级中向上移动，而退休前和退休年龄的人则向下移动。老年人（75岁及以上）倾向于晋升。从性别差异来看，女性的人口效率高于男性。此外，几乎所有年龄组中，女性的向上流动都占主导地位。这些发现表明，总体移民行为可能因性别而异，这一点在之前关注其他国家城市等级制度的移民研究中很少受到关注。此外，本研究还根据观察到的模式讨论了政策含义，例如老年人的流动对各个层级社会保障的影响，以及妇女向上流动模式与未来生育率下降之间的可能关系。