Life expectancy at birth is past 84 in Japan and only in the mid-fifties in Chad and Nigeria. A child's chances of finishing school, earning a decent wage or reaching old age still depend heavily on where on the map she happens to be born. This level asks a geographer's question: why is prosperity spread so unevenly across space, and why do the gaps last?

An old answer: environmental determinism

A century ago many geographers had a simple answer. Environmental determinism is the claim that the physical environment, especially climate, fixes the character and success of societies. The American geographer Ellen Churchill Semple opened her 1911 book Influences of Geographic Environment with the words "Man is a product of the earth's surface." Ellsworth Huntington argued that temperate climates produced energetic, inventive peoples while hot climates produced idleness. These ideas flattered Europeans and were used to justify colonial rule. They also rested on weak evidence and circular reasoning: whatever a society achieved was credited to its climate after the fact.

The French school of geography, led by Paul Vidal de la Blache, offered a different view. Possibilism holds that the environment sets limits and offers options, but people choose among them according to their culture, technology and history. Most geographers today hold some version of this view: mountains, rivers, soils and diseases matter, but they do not dictate.

Geography still matters

Rejecting determinism does not make the land irrelevant. Landlocked countries usually face higher trade costs, because their goods must cross a neighbor's territory to reach a port. Tropical regions carry a heavier burden of diseases such as malaria. In Guns, Germs, and Steel (1997), Jared Diamond argued that Eurasia's long east-west axis helped crops and animals spread, since places at similar latitudes share similar climates and day lengths, while the north-south layout of the Americas and Africa slowed the spread of farming. Critics accept parts of the argument but warn that it can drift back toward determinism and pays too little attention to human choices, especially conquest and colonial extraction. A rival school, associated with the economists Daron Acemoglu and James Robinson, argues that institutions, the rules that either protect people or prey on them, explain more of the gap than geography does.

Core and periphery

Another family of explanations focuses on relationships between places. The sociologist Immanuel Wallerstein's world-systems theory, set out in the 1970s, divided the world economy into a core, a periphery and a semi-periphery. In core-periphery models, wealthy core regions draw raw materials, labor and profits from poorer peripheral ones, so the poverty of one area is partly a product of its links with another, not simply a starting condition. Supporters point to colonial economies built to export a single crop or mineral. Critics point to South Korea and Taiwan, which moved from the periphery toward the core within two generations, as evidence that the system is less fixed than the theory suggests.

In 1980 a commission chaired by the former West German chancellor Willy Brandt published a report whose cover map showed a line dividing the richer North from the poorer South. It was a powerful image, but it has aged: China, the Gulf states and much of East Asia have changed position since, and many geographers now prefer to speak of a spectrum rather than a single divide.

Inequality at every scale

Inequality is not only a matter of rich and poor countries. Uneven development means that growth concentrates in some regions while others lag behind, and it happens inside nations, cities and even neighborhoods. Capital cities and ports often pull ahead, attracting investment and ambitious young people, which leaves rural or old industrial regions older and poorer. The Swedish economist Gunnar Myrdal called this cumulative causation, a process in which success breeds more success and decline feeds on itself. Regional gaps also feed political anger.

Worked example: Italy's north and south

Italy has one of Europe's sharpest internal divides. The south, known as the Mezzogiorno, has long had lower incomes, higher unemployment and heavier emigration than the industrial north around Milan and Turin. Each theory in this level offers part of an explanation. Determinists once blamed the southern climate, a claim few now accept. Possibilists point to real constraints, such as mountainous terrain and distance from Europe's main markets. Core-periphery thinkers note that after unification in 1861, the south supplied labor and customers for northern industry. Institutional accounts stress concentrated land ownership, weak local government and organized crime.

From 1950 to 1984 the Italian state poured money into the south through a development fund, the Cassa per il Mezzogiorno, building roads, dams, irrigation and factories. Some projects helped, but others became known as cathedrals in the desert: huge plants that created few links with local firms and sometimes fed corruption. The income gap narrowed for a time and then stalled. The lesson is that inequality between places rarely has a single cause, and that money alone cannot close it without attention to the people and institutions on the ground.