This paper answers the following two questions: (1) In the data, do we find a dilution effect of population growth also on per-capita human capital investment? If yes, (2) How can we use this observation to explain theoretically the existence of a differential impact of population change on economic growth across countries? In the first part of the article we document empirically considerable cross-country heterogeneity of a dilution effect of population growth with regards to the process of per-capita human capital formation. And we observe that, depending on the dilution effect observed in a given country, population growth may be relevant — either positively or negatively — for economic growth. In the second part of the paper we use these results to build a multi-sector growth model which, by allowing for a country specific dilution effect of population growth on per-capita human capital formation, is capable of accounting for the non-monotonous correlation between demographic and economic growth rates in the long-run.