Question

countries with high fertility levels face serious problems, how does it fit into the solow growth model?

Answer 1

Their main problem due to the Solow model is that their graph includes productivity. To say more, the line that describes the cost of productivity is straight, but those lines that describe the productiveness have a form of $\sqrt X$ function. So increasing the number of people make the number of capital on one person decrease, which decrease productiveness.