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.

There is a universal trend of declining fertility since 1970s in virtually all countries of the world despite their large differences in the levels of fertility. This trend is likely to continue in the future and fertility rates of the countries are predicted to convergence to replacement level by the end of this century. Cross-country data indicate that a higher level of economic and social development leads to a lower level of fertility. Given HDI representing a country’s economic and social development level, statistical estimations based on data of 109 countries with over 5 million people in 2010 demonstrate a strong reverse relationship between HDI and TFR among the countries: an increase in HDI is associated a corresponding decrease in TFR; and when HDI is greater than 0.71, TFR drops to below replacement level. This relationship can be explained by an array of interplays of economic and social factors, among which a significant linkage is with the variable of urbanization.

Based on the data from a sampling survey of rural families of aged parents(50+) with one child across 5 counties(or districts),China,we have found that over half of the families have not become "richer with fewer children",one of the most famous propaganda slogans in family planning work.The daily life of aged parents in these families has not been secured.Over half of these families have an income lower than the national poverty line;nearly 80% of the parents over 50 years old are living in some degree of economic shortages.Given their contributions to population control in China,more economic support should be provided to them.