In advanced countries, the trend of declining birthrates and aging populations is progressing, with Japan being particularly advanced in this regard. The government aims to increase the future labor force by raising the birthrate to ensure a sustainable social security system. To achieve this, proactive childcare support policies are being implemented. In economics, how are population and birthrate endogenization modeled and analyzed? This chapter explains the methods for setting up an endogenous fertility model. Furthermore, based on this model, we will examine the factors that contribute to raising the birthrate and the impact of childcare support policies on the birthrate.