Fiscal Dynamics in Japan under Demographic Pressure

Abstract: Japan's population is shrinking, the share of working-age people is falling, and the number of elderly is growing fast. These trends squeeze public finances from both sides--fewer people paying taxes and more people drawing on pensions and healthcare. Policy discussions often focus on one fix at a time, such as raising taxes, reforming pensions, or boosting productivity. However, these levers interact with each other through feedback loops and time delays that are not yet well understood. This study builds and calibrates an integrated system dynamics model that connects demographics, labor supply, economic output, and public finance to explore two questions: (RQ1) What feedback structure links demographic change to fiscal outcomes, and how do different policy levers work through that structure? (RQ2) Which combinations of policies can stabilize key fiscal indicators within a meaningful timeframe? The model, grounded in official statistics, tracks historical trends reasonably well. Policy experiments show that productivity improvements and controlling per-person costs offer the most effective near-term relief, because they act quickly through revenue and spending channels. In contrast, raising fertility actually worsens the fiscal picture in the medium term, since it takes decades for newborns to grow up and join the workforce. A combined scenario pairing moderate productivity gains with moderate cost control nearly eliminates the deficit by 2050. These findings underscore the importance of timing when evaluating demographic policy. Stabilizing finances within a practical timeframe requires levers that improve the budget directly, rather than those that work through slow demographic channels. The model serves as a transparent testing ground for designing time-aware fiscal policy packages in aging, high-debt economies.