This essay is adapted from Why Nations Fail: The Origins of Power, Prosperity and Poverty, published this week. For more from these authors, see their blog.

If you start in the city center of Nogales, Santa Cruz [Arizona] and walk south for a while, at some point you see houses become much more run down, streets turn decrepit. You have crossed the Mexican border into Nogales, Sonora. Though the two cities are made of the same cloth and were once united, now there are sharp differences between the two. Those in the north are about three times as rich, have access to much better health care, stay in school much longer and of course take part in a much more democratic political process than their cousins in the south. The differences between the two halves of Nogales are a micro, tiny version of huge differences in prosperity and living standards we see around the world. Take Mexico as a whole, for example: it has less than one quarter of the GDP per capita of the United States. Take Peru; it has about one seventh of the GDP per capita of the United States. Or take Ethiopia, Haiti, Somalia or the Congo, each of [which] has less than one thirtieth of the GDP per capita of the United States. Our thesis is that these differences are the outcome of different economic and political institutions which lead to very different incentives.

Though history bears out the defining role of institutions in shaping prosperity and poverty, most social scientists and experts have emphasized different factors. One of the most widely accepted alternative theories of world inequality is the geography hypothesis, which claims that the great divide between rich and poor countries is created by geographical differences. Many poor countries, such as those of Africa, Central America, and South Asia, are between the tropics of Cancer and Capricorn. Rich nations, in contrast, tend to be in temperate latitudes. This geographic concentration of poverty and prosperity gives a superficial appeal to the geography hypothesis, which is the starting point of the theories and views of many social scientists and pundits alike. But this doesn’t make it any less wrong.

As early as the late eighteenth century, the great French political philosopher Montesquieu noted the geographic concentration of prosperity and poverty, and proposed an explanation for it. He argued that people in tropical climates tended to be lazy and to lack inquisitiveness. As a consequence, they didn’t work hard and were not innovative, and this was the reason why they were poor. Montesquieu also speculated that lazy people tended to be ruled by despots, suggesting that a tropical location could explain not just poverty but also some of the political phenomena associated with economic failure, such as dictatorship.

The theory that hot countries are intrinsically poor, though contradicted by the recent rapid economic advance of countries such as Singapore, Malaysia, and Botswana, is still forcefully advocated by some, such as the economist Jeffrey Sachs. The modern version of this view emphasizes not the direct effects of climate on work effort or thought processes, but two additional arguments: first, that tropical diseases, particularly malaria, have very adverse consequences for health and therefore labor productivity; and second, that tropical soils do not allow for productive agriculture. The conclusion, though, is the same: temperate climates have a relative advantage over tropical and semitropical areas.