Abstract: Population momentum and population inertia are connected concepts. Both are used to describe the phenomenon that the population will continue growing or decreasing before reaching a stationary population when the fertility reaches the replacement level. However, there are also differences. The population momentum is virtual and can be estimated for any year, while population inertia is a specific demographic phenomenon that only occurs after the population's fertility reaches the replacement level. The Lag effect of fertility on generation replacement is the fundamental cause of population inertia and determines that the standard period of inertial period is equal to the difference between the population's life expectancy at birth and the mean age of childbearing. The length of an actual inertial period of the population is affected by the fertility level during the inertial period and the size of migration. The amplitude of population growth or decrease during the inertial period is affected by the fertility rates in the decades before entering the inertial period. Empirical analysis shows that China, Japan, South Korea and Germany have already undergone the complete processes of population inertial growth which have lasted for 30-50 years. The UK, France, Thailand and Brazil are in the process of population inertial growth. So far, no country in the world has entered the process of negative inertial growth of population.

Keywords: Population Momentum, Population Inertial Change, Population Inertial Growth, Population Inertial Decrease, Low Fertility

摘要： 人口惯性变动与人口动量是两个既有联系又有区别的概念，人口惯性变动是指人口增减转变滞后于生育率高低转变的现象，人口动量反映的是假设生育率立刻下降或上升到更替水平后人口增加或减少的幅度，是一种虚拟结果，实际发生的人口惯性变动现象包含的信息比人口动量更丰富。生育的代际更替滞后效应是导致人口惯性变动的根本原因，并决定了人口惯性变动标准期。人口实际惯性变动期长短受惯性变动期间生育率高低和移民规模大小的影响，人口惯性增减幅度则受进入惯性变动期之前数十年间生育水平的影响。实证分析显示，中国、日本、韩国、德国已完成人口惯性增长全过程，实际惯性增长期长达30~50年；英国、法国、泰国、巴西正处于人口惯性增长过程中，大约在21世纪中叶前后才会结束惯性增长。目前，世界上尚未有任何国家进入人口惯性负增长过程。

关键词: 人口动量, 人口惯性变动, 人口惯性增长, 人口惯性负增长, 低生育率