本文以城市人口密度作为切入点，研究人口集聚与城市经济发展内在逻辑。主要回答问题：（1）人口集聚能否促进城市经济发展？对不同特征城市的影响是否存在差异？（2）人口过度集聚是否会对经济增长产生负面影响？（3）集聚效应和拥挤效应对城市经济发展的影响机制是什么？对于中国城市化应当如何发展的问题，长期以来就存在着“大城市重点论”和“小城镇重点论”的争议。在对已有文献进行系统梳理的基础上，本研究基于人口集聚带来的集聚效应和拥挤效应两个维度构建了分析框架。就集聚效应而言，主要从知识溢出、本地市场、劳动力池机制展开讨论；就拥挤效应而言，从交通拥堵、环境污染、住房成本方面进行分析。此外，结合城市异质性特点，进一步研究人口集聚对城市经济发展在不同特征的城市之间存在的差异。本文使用城市层面数据进行回归分析，发现人口集聚对城市经济发展具有显著的促进作用。由于人口集聚与城市经济发展的联系存在反向因果、遗漏变量以及测量误差等内生性问题，本研究使用中国省级层面实施的统一城乡户籍改革政策这一准自然实验，同时引入“城市建筑日照间距标准”作为城市人口密度的工具变量，进一步建立两者之间的因果关系。进一步地，本文使用上市公司数据、家庭调查数据、劳动力调查数据等微观层面证据验证集聚效应对于经济发展的三个机制假说，分别从企业创新能力、企业从事服务业决策与劳动力家庭服务型消费投入、劳动力就业匹配与技能培训进行分析，打开人口集聚对于城市经济发展影响的黑箱。研究发现，人口集聚能够促进城市经济增长，城市建成区就业人口密度每增加1万人/平方公里，城市人均实际生产总值增长58.8%。在大城市和特大城市中，人口集聚对于经济发展的影响效应更强，而在超大城市中这一集聚效应并不显著，说明在超大城市中集聚效应无法抵消拥挤效应的负外部性从而实现净规模收益。微观证据表明，人口集聚能够显著提升城市上市公司的技术创新能力，促进生活性服务企业发展，提高家庭生活性服务业消费支出，增强劳动力就业岗位的匹配程度和工作技能。此外，本文发现当人口密度达到一定程度时，人口集聚对经济发展的促进作用会被拥挤效应不断削弱，即人口密度与经济发展呈现倒U型关系。机制分析发现，拥挤效应通过降低劳动者工作效率、损害个体健康资本、挤出家庭消费支出等微观渠道阻碍城市经济发展。综合考虑集聚效应和拥挤效应后，实证估算得出城市最优就业人口密度为6325人/平方公里。

AbstractThe population has an important role in urban agglomeration for economic and social development. As the main spatial carrier that promotes future economic growth, the city carries the main innovation, production and life activities of human beings, and the population gathering has brought rich human capital to urban development, which makes it possible to extend the industrial chain and deepen the division of labor. Under the background of low fertility, population aging problems, how to understand the impact of population gathering is especially crucial. This paper uses urban population density as interpretation variables to study the role of population agglomeration on urban economic development. The main research questions include: (1) Can population agglomeration promote urban economic development? Is there a difference in the impact on cities with different characteristics? (2) Does the excessive population aggregation have a negative impact on economic growth? (3) What is the impact mechanism of agglomeration effect and congestion effect on urban economic development?Regarding the development path of China's urbanization, there is always a discussion about the priority development of big cities or small towns. The analysis framework was constructed based on the two dimensions of the aggregation effect and congestion effect. In terms of agglomeration effect, this paper mainly discusses the mechanism of knowledge spillover, local market, and labor pool; in terms of congestion effects, this paper analyzes the mechanism of traffic congestion, environmental pollution, and housing cost. In addition, combined with the characteristics of urban heterogeneity, this paper further studies the differences of population agglomeration on urban economic development among different cities.First, based on city-level regression analysis this paper finds that the population agglomeration has a significant role in promoting urban economic development. Due to endogeneity problems, such as reverse causal, omitted variables and measurement errors between population agglomeration and urban economic development, this paper exploits the unified urban-rural household registration reform policy implemented in China as a natural experiment, and introduces ‘sunlight spacing standard for urban architecture’ as the instrumental variable to establish the causal effects of population density on urban economic development. Further, using rich micro-level data including listed firm data, household survey data and labor force survey data, this paper verifies the three mechanism hypotheses of the agglomeration effect on economic development, specifically involving corporate innovation, development of service sector from enterprise and household perspectives, and employment matching for labor force, which helps us to open the black box of the impact of population agglomeration on urban economic development. In addition, this paper takes the Economic and Technological Development Zone in Z Municipal as a research case, which provided sufficient realistic evidence for this study. Finally, through analyzing labor force activities and household decisions, including commuting and leisure time utilization, physical and mental health, and consumption and savings, this paper discusses the urban congestion effects from the three aspects of traffic congestion, environmental pollution, and housing costs. On top of all, this paper finds out the urban optimal employed density, or the peak value of net economic benefits.Empirical evidence shows that population agglomeration can significantly promote urban economic growth. Urban employed population density in built-up area increased by 10,000 people per square kilometer, the urban per capita real GDP grew by 58.8 percent. Besides, the agglomeration effect is stronger in large cities, but the agglomeration effect is not significant in oversized cities, indicating that the agglomeration effect cannot offset the negative externality of the congestion effect to achieve net scale benefits in oversized cities. Micro evidence shows that population agglomeration can significantly improve the technology innovation capability of listed companies, promote the development of living service industry, enhance the matching efficiency of employment positions and labor skills. Furthermore, this paper finds that when the population density reaches a certain level, the positive effects of population agglomeration will be weakened by the congestion effect, ie, the population density and economic development present an inverted U-shaped relationship. Mechanism analysis demonstrates that congestion effects hinder urban economic development by reducing labor efficiency, harming individual health capital, and crowding out household consumption expenditure. Considering agglomeration and congestion effects simultaneously, our empirical results show that urban optimal employed population density is 6,325 people per square kilometer.Finally, this paper suggests that the leading role of large cities in economic development should be further emphasized and government should construct a modern urban circle, promoting the coordinated development of large, medium, and small-sized cities. At the same time, it is necessary to continuously deepen the reform of the household registration system, actively promote the construction of new cities, create livable, innovative, smart, green, humanistic, and resilient cities, making the best of population agglomeration in promoting urban economic development.