Due to the positive correlation between population size and technological innovation, there exists a positive externality in fertility, which is intertemporal and inseparable across regions. Since the benefits of high fertility are reaped at the national level, the central government has the principal responsibilities to implement pronatalist policies. Policy design deviating this principle would distort the behaviors of local governments. In this context, a well-designed pronatalist policy should clearly define the responsibilities between the central and the local governments to achieve a mechanism with incentive compatibility at both the micro and macro level. The core of pronatalist policy is to resolve the conflicts between childbirth and female labor market participation and to prioritize those that both facilitate childbearing and improve women's competitiveness in the labor market. Personal income tax deduction is one of the policies that meets the above criteria. The top-level design should complete the policy framework to effectively include women of childbearing age and implement targeted policies based on the characteristics of different groups.

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.

Changes in people's marital and fertility intentions and behaviors play a significant role in declining fertility rates. Based on data from a specialized survey, this paper investigates the intentions and determinants of marriage and fertility within distinct childbearing-age groups, concerning non-marriage or delayed marriage, childlessness or delayed childbearing, and low fertility, under the background of current fertility policies. It finds that the majority of individuals in these groups still wish to marry and have children. The main reasons for the divergence between their intentions and behaviors are the rising costs of marriage and childbearing, including economic and time costs. Furthermore, the impact of cultural and public opinion cannot be overlooked. Regarding the Three-child policy and supporting measures, over twenty percent of the married respondents consider them effective in boosting childbearing intentions. Reducing educational costs, providing child-rearing subsidies, and ensuring parental leave are the common policy demands. Therefore, it is crucial to reduce the costs of child-rearing for families through diversified economic support, establish a comprehensive lifecycle marital and childbearing support policy system, and convey positive marital and childbearing concepts through cultural development.

Analysis of the China Family Panel Studies (CFPS) data reveals that fertility motivations mediate the impact of internet use on fertility intentions. This mediating role operates in two directions. First, internet use diminishes family-oriented fertility motivations, including family legacy and economic utility motivation, suppressing fertility intentions. Second, internet use enhances individual-oriented fertility motivations, particularly emotion-driven motivation, promoting fertility intentions. Overall, the inhibiting role played by fertility motivation is more pronounced in the influence of internet use on fertility intentions. The mediating effect accounts for 11.251% of the total effect. Further, the impact of internet use on fertility motivation differs across cohorts, genders, urban-rural residences, and education levels. The impacts of different online behaviors on fertility motivation are also different. Exploring the mediating role of fertility motivation contributes to a deeper understanding of the mechanisms through which internet use influences fertility intentions, offering novel insights for addressing low fertility rates in the internet era.

With the development of the ageing process and the advancement of new-type urbanization, the size of empty-nest older adults gradually increases. The expansion of the Internet has enabled more older adults to access the Internet. Based on the data from the China General Social Survey (CGSS) from 2010 to 2018, this study estimates the age-period-cohort effects and the impacts of Internet use on the health status of empty-nest older adults. The results show that there are period and cohort effects, but not age effects, on the changes in the mental health of empty-nest older adults. However, there are age and period effects, but not cohort effects, on the changes in physical health. Internet use has a significant positive impact on the physical and mental health of empty-nest older adults, mainly through social and learning pathways. The impact of Internet use on the health of empty-nest older adults is greater for those aged 75 and above, as well as for rural empty-nest older adults. Compared to non-empty-nest older adults, there is a greater impact of Internet use on the health of empty-nest older adults. We propose promoting Internet use among the empty-nest older adults and building an age-friendly digital society.

The population ageing in China has become increasingly profound, and Actively Responding to Population Ageing has become a national strategy. While the older adults at age 60-69 (i.e., the young old adults) have great human resources to be utilized, their labor force participation levels have not been thoroughly examined. Based on data from the censuses and 1% population sampling surveys from 2000 to 2020, this study describes the demographic characteristics and labor force participation features of the young old adults in China from the perspectives of urban-rural areas, regions, gender, education and health status. The study reveals that the size of the young old adults in China is rapidly growing, with continuous improvements in education and health human capital. However, the efficiency of utilizing the human resources of the young old adults remains low. The young old workers in rural and remote areas primarily engage in agricultural activities, with a relatively single employment structure. Although the young old adults in urban and developed areas have more job opportunities, they mostly engage in low-end labor with low incomes and poor social insurance. The young old women have great potential in terms of human resources, but they face more employment obstacles at institutional, socio-cultural and personal cognitive levels.

Based on the panel data of China's prefecture-level cities in 2000, 2010, and 2020, this paper examines the spatial effects of population ageing on technological innovation and explores the heterogeneous impacts of ageing on technological innovation at different stages of social and economic development. The analytical results of the spatial Durbin model exhibit a robust inverted U-shaped relationship between population ageing and local technological innovation, as well as a U-shaped relationship between population ageing and neighbors' innovation. Furthermore, the threshold regression models indicate that, at the stage of low urbanization rate and insufficient supply of health services, population ageing imposes a significantly negative impact on technological innovation. With higher urbanization and a more adequate supply of health services, population ageing would stimulate innovation. Therefore, against the backdrop of the irreversible ageing process, promoting the urbanization process and improving the health care system will effectively attenuate the discouraging effects of ageing on innovation.

Paying attention to the physical and mental health of Chinese older adults is of great significance for promoting the implementation of the National Strategy for Actively Responding to Population Ageing and the Healthy China strategy. By applying the fixed effect model and cross-lagged panel model to the data of the China Health and Retirement Longitudinal Study (CHARLS) 2013, 2015, and 2018, this paper analyzes the causal relationship between physical disability and depressive symptoms among Chinese older adults. The results show that there is a significant positive correlation between physical disability and depressive symptoms during the same period. After controlling for unobserved confounding factors, the deterioration and improvement of physical disability will significantly affect changes in depressive symptoms. Using the cross-lagged panel model, we found a bidirectional causal relationship between physical disability and depressive symptoms among older adults. By utilizing three physical disability measurement indices, namely the Nagi Index, IADL Index, and ADL Index, it was discovered that the bidirectional causal relationship between them remains significant. Therefore, it is necessary to enhance the prevention and rehabilitation of physical disability among Chinese older adults and to construct a comprehensive health service system for older adults that encompasses various aspects, including physical and mental health.

Studying population flow patterns is essential for comprehending regional population changes and economic and social development trends. Using data from the sixth and seventh national population censuses to classify China's interprovincial population flows into four types: rural-urban, rural-rural, urban-urban, and urban-rural, this paper analyzes the spatial characteristics of population flow networks and their influencing factors using a spatial autoregressive negative binomial model. The results show that: The proportion of rural-urban and urban-urban population flows has increased, while the proportion of rural-rural population flows has decreased. Rural-rural and rural-urban population flows tend to move from west to east, while urban-urban and urban-rural population flows are diverse and bi-directional. China's population center is shifting from a single city (province) to an urban agglomeration. While economic factors play a significant role in rural-urban and rural-rural population flows, the factors influencing urban-rural and urban-urban population flows are complex and diverse. The degree of influence of economic factors on these flows is weakening.