sentences:  Garett Jones (2013) builds upon Kremer's O-ring theory to explain why differences in worker skills are associated with "massive" differences in international productivity levels despite causing only modest differences in wages within a country.For this purpose, he distinguishes between O-ring jobs - jobs featuring high strategic complementarities in terms of skill - and foolproof jobs - jobs characterized by diminishing returns to labor - and assumes both production technologies to be available to all countries.He then goes on to show that small international variations in average worker skill per country result in both large international and small intra-national income inequality.