How does urban cluster population agglomeration enhance new quality productive forces? Empirical evidence from 19 urban clusters in China

Urban clusters possess a distinct "system integration advantage" that exceeds that of individual cities, positioning them as the optimal spatial platform for cultivating New Quality Productive Forces (NQPF). Drawing on panel data from 221 cities across 19 urban clusters in China from 2011 to 2022, this study investigates the impact of urban cluster population agglomeration (UCPA) on NQPF and explores its underlying mechanisms. The empirical results demonstrate that UCPA significantly promotes the development of NQPF, a conclusion that remains robust across multiple validation tests. Heterogeneity analysis reveals that this promotional effect varies across urban location, city scale, and urban cluster type. Specifically, the effect is more pronounced in peripheral cities, small-to-medium-sized cities, mega urban clusters, and intra-provincial urban clusters. Mechanism analysis indicates that UCPA enhances NQPF primarily through two channels: fostering technological innovation and expanding market demand. Furthermore, threshold effect analysis reveals a nonlinear relationship, whereby the positive impact of agglomeration is significantly amplified once a critical threshold is surpassed—an effect not observed in single-city contexts. These findings offer important policy implications for rationally guiding population distribution, unlocking the spatial agglomeration dividend across broader regions, and maximizing the role of coordinated regional development—centered on urban clusters—in advancing NQPF.