Although the problems created by exceeding Earth’s carrying capacity are real, a too-small population also creates problems. The convergence of a nation’s population into small areas (i.e., cities) via processes such as urbanization can accelerate the evolution of a more advanced economy by promoting new divisions of labor and the evolution of new industries. The degree to which population density contributes to this evolution remains unclear. To provide insights into whether an optimal “threshold” population exists, we quantified the relationships between population density and economic development using threshold regression model based on the panel data for 295 Chinese cities from 2007 to 2019. We found that when the population density of the whole city (urban and rural areas combined) exceeded 866 km⁻², the impact of industrial upgrading on the economy decreased; however, when the population density exceeded 15,131 km⁻² in the urban part of the cities, the impact of industrial upgrading increased. Moreover, it appears that different regions in China may have different population density thresholds. Our results provide important insights into urban economic evolution, while also supporting the development of more effective population policies.

Sustainable development has attracted widespread attention worldwide, and the circular economy has become one of the essential policies of many countries. Small and medium-sized enterprises are important drivers of world economic growth and can significantly impact the environment. Therefore, SMEs are critical players in implementing a circular economy as the basis for creating a sustainable society. Although a wealth of research on SME environmental management issues can be found in the literature, more must be known about the infusion of green practices in SMEs. The primary purpose of this study is to explore the green practice infusion of Taiwanese SMEs, a context that is particularly relevant due to Taiwan’s strong focus on environmental sustainability and its circular economy industrial development policy. Through a questionnaire survey, this study examined the factors that influence green practice infusion behavior in Taiwanese SMEs and the impact of green practice infusion on circular economy performance. The findings show that the relative advantages and compatibility of the circular economy, organizational support, human resource quality, regulatory pressure, and government support significantly impact the green practice infusion of Taiwanese SMEs. The effects of complexity, customer pressure, and environmental uncertainty on SMEs’ infusion of green practices are not statistically significant. Circular economy performance is positively correlated with green practice infusion. This study can broaden the research scope of SMEs’ environmental management and contribute to a deeper understanding of SMEs’ green practice infusion and circular economy.