Book excerpt: This paper offers a thesis as to why the US overtook the UK and other European countries in the 20th century in both aggregate and per-capita GDP, as a case study of recent models of endogenous growth where human capital is the "engine of growth". The conjecture is that the ascendancy of the US as an economic superpower owes in large measure to its relatively faster human capital formation. Whether the thesis has legs to stand on is assessed through stylized facts indicating that the US led other OECD countries in schooling attainments per adult population over the 20 century, especially at the secondary and tertiary levels. While human capital is viewed as the direct facilitator of growth, the underlying factors driving the US ascendancy are linked to the superior returns the political-economic system in the US has so far offered individual human capital attainments, both home-produced and imported.