This paper explores how population growth can affect economic growth in a model where human and physical capital are important for creating new human capital. The researchers found that population growth can have both positive and negative effects on economic growth, depending on how human and physical capital interact. They also discovered that income growth can happen even with a stable population, and that per-capita income is linked to per-capita human capital. Overall, the level of per-capita income is proportional to the amount of human capital individuals have.