This chapter considers two types of models and examines how fertility is determined. One model treats policy variables as exogenous, while the other has policy determined by a majority voting system. The former economy has a unique steady state, which can be either stable or unstable. Consequently, fertility rates either converge to or diverge from this steady state. The stability of fertility dynamics is influenced by the child allowance per child. However, in the latter economy, a high ratio of older people leads to greater allocations for pension benefits because older people have an influential voice; that is, the government must enact policies favored by older people. Conversely, if younger people have an influential voice in government policy, the government implements policies favored by the younger population, such as child allowances. This economy results in two steady states: one with low fertility and the other with high fertility. The fertility dynamics converge to the low-fertility steady state under certain conditions if the ratio of older people is high. This result illustrates the phenomenon prevailing in contemporary Japan.