Persistently declining fertility has become a major demographic issue in post-industrial societies. After the completion of the first demographic transition, fertility rates in many countries did not stabilize around the replacement level, but continued to decline and exhibited characteristics of the second demographic transition. Household fertility choices, shaped by constraints of cost, time, and risk, have gradually diverged from society's long-term needs for labor supply, market capacity, and balanced population structure. Explaining the mechanism underlying this divergence is the central concern of this paper. From the perspective of the deepening social division of labor, this paper combines theoretical analysis with model-based deduction and traces the historical evolution from traditional agricultural society to the early stage of industrialization and then to the post-industrial stage. It examines the relationships among household functions, firm behavior, and population-scale demand under different forms of division of labor, and reveals the mechanism through which a structural contradiction emerges between household fertility choices and societal population demand.

The study finds that, in traditional agricultural society, the family performed multiple functions of production, consumption, old-age support, and risk protection. A larger household size helped strengthen household production capacity, labor organization, and intergenerational support, making relatively high fertility economically rational under conditions of low productivity and limited external support. In the early stage of industrialization, labor-intensive production and the expansion of intra-firm division of labor increased social demand for labor. Meanwhile, rising household income, declining mortality, and relatively low childrearing costs jointly sustained high fertility. Thus, expanding labor demand and high household fertility formed a positive coupling relationship during the transitional phase of the first demographic transition. In the post-industrial stage, however, the continuous deepening of the social division of labor has externalized household functions of production, security, and care, weakened individuals' economic and functional dependence on the family, and increased the time cost, opportunity cost, and quality-investment pressure of childbearing. As a result, the household's optimal number of children declines. At the same time, firm operation and socioeconomic development still rely on labor supply, consumer markets, and product-variety expansion supported by population size. Society therefore retains a long-term demand for population stability and structural balance. This gives rise to a structural divergence between households' rational low-fertility choices and societal population demand.

The contribution of this paper lies in explaining persistent low fertility and the second demographic transition within the framework of the evolving social division of labor. It further points out, from the perspective of firms' dependence on population size and market capacity, that there is a systematic mismatch between the social benefits of population reproduction and the private costs borne by households. Addressing this contradiction requires shifting fertility costs from being borne solely by households to being reasonably shared by the state, firms, and households. In particular, firms should assume greater responsibility in the fertility support system, so that the benefits and costs of childbearing can be more fairly distributed across social actors and a more coordinated relationship can be established between household fertility decisions and the long-term needs of social development.