Population aging puts pressure on the financing of pension systems in many developed eonomies. Higher fertility would reduce the speed at which a population ages, and thus ease the financing pressure. The positive fiscal externality of childbearing decisions is well known in a single-country context. I perform a quantitative investigation of this externality in a multi-country context, to investigate the role of capital markets integration. On a sample of 14 European countries confronted with population aging, I find with simulations that the fiscal gains from an increase in fertility are on average 1.3 larger with integrated capital markets than with separated capital markets in 50 years, while the GDP per capita gains are 5.0 larger. Positive fertility shocks indeed depress the capital-labor ratio, increase the interest rate and thus attract foreign investors, when capital markets are open. As an example of policy implication, the plans for further integration of capital markets in the European Union provide an additional motivation for public policies designed to promote fertility in this region of the world, and vice-versa.