If the geography hypothesis cannot explain differences between the north and south of Nogales, or North and South Korea, or those between East and West Germany before the fall of the Berlin Wall, could it still be a useful theory for explaining differences between North and South America? Between Europe and Africa? Simply, no…
Whatever the drawbacks of the Inca and Aztec empires were in 1532, Peru and Mexico were undoubtedly more prosperous than those parts of the Americas that went on to become the United States and Canada. North America became more prosperous precisely because it enthusiastically adopted the technologies and advances of the Industrial Revolution. The population became educated and railways spread out across the Great Plains in stark contrast to what happened in South America. This cannot be explained by pointing to differential geographic endowments of North and South America, which, if anything, favored South America.
Inequality in the modern world largely results from the uneven dissemination and adoption of technologies, and Diamond’s thesis does include important arguments about this. For instance, he argues, following the historian William McNeill, that the east–west orientation of Eurasia enabled crops, animals, and innovations to spread from the Fertile Crescent into Western Europe, while the north–south orientation of the Americas accounts for why writing systems, which were created in Mexico, did not spread to the Andes or North America. Yet the orientation of continents cannot provide an explanation for today’s world inequality. Consider Africa. Though the Sahara Desert did present a significant barrier to the movement of goods and ideas from the north to sub-Saharan Africa, this was not insurmountable. The Portuguese, and then other Europeans, sailed around the coast and eliminated differences in knowledge at a time when gaps in incomes were very small compared with what they are today. Since then, Africa has not caught up with Europe; on the contrary, there is now a much larger income gap between most African and European countries.
It should also be clear that Diamond’s argument, which is about continental inequality, is not well equipped to explain variation within continents—an essential part of modern world inequality. For example, while the orientation of the Eurasian landmass might explain how England managed to benefit from the innovations of the Middle East without having to reinvent them, it doesn’t explain why the Industrial Revolution happened in England rather than, say, Moldova.