Researchers at Harvard's Center for International Development (CID) found in 2001 that only three tropical economies — Hong Kong, Singapore, and part of Taiwan — were classified as high-income by the World Bank, while all countries within regions zoned as temperate had either middle- or high-income economies.
There are exceptions, for example, Russia is less well off than the United States, or even Australia; the latter country is wealthier than Southern Europe. Also, within Russia, Moscow and especially St. Petersburg are wealthier than, for example, Siberia. Similarly, Germany's poorest regions are situated in the North East, and in general, the south of the UK is wealthier than the north. The oil-rich countries of the Middle East present self-explanatory exceptions, being wealthier than Central Asia, for comparison. The wealthiest nations of Central America are generally those closest to the equator, namely Panama and Costa Rica. Within the Far East, the south of Korea is much wealthier then the north, and Mongolia is poorer than China. And in south-east Asia, the richest nations are Malaysia and Thailand, straddling the equator.
Most of the recent studies use national GDP per capita, as measured by the World Bank and the International Monetary Fund, as the unit of comparison. Intra-national comparisons use their own data, and political divisions such as states or provinces then delineate the study areas.
The 19th century historian Henry Thomas Buckle wrote that "climate, soil, food, and the aspects of nature are the primary causes of intellectual progress,--the first three indirectly, through determining the accumulation and distribution of wealth, and the last by directly influencing the accumulation and distribution of thought, the imagination being stimulated and the understanding subdued when the phenomena of the external world are sublime and terrible, the understanding being emboldened and the imagination curbed when they are small and feeble."
Physiologist Jared Diamond was inspired to write his Pulitzer Prize-winning work Guns, Germs, and Steel by a question posed by a New Guinean politician: why were Europeans so much wealthier than his people? In this book, Diamond argues that the Europe-Asia land mass is particularly favorable for the transition of societies from hunter-gatherer to farming communities. This continent stretches much further along the same lines of latitude than any of the other continents. Since it is much easier to transfer a domesticated species along the same latitude than it is to move it to a warmer or colder climate, any species developed at a particular latitude will be transferred across the continent in a relatively short amount of time. Thus the inhabitants of the Eurasian continent have had a built-in advantage in terms of earlier development of farming, and a greater range of plants and animals from which to choose.
Diamond notes that modern technologies and institutions were designed primarily in a small area of northwestern Europe. After the Scientific Revolution in Europe in the 1500s, the quality of life increased and wealth began to spread to the middle class. This included agricultural techniques, machines, and medicines. These technologies and models readily spread to areas colonized by Europe which happened to be of similar climate, such as North America and Australia. As these areas also became centres of innovation, this bias was further enhanced. Technologies from automobiles to power lines are more often designed for colder and drier regions, since most of their customers are from these regions.
The book goes on to document a feedback effect of technologies being designed for the wealthy, which makes them more wealthy and thus more able to fund technological development. He notes that the far north has not always been the wealthiest latitude; until only a few centuries ago, the wealthiest belt stretched from Southern Europe through the Middle East, northern India and southern China. A dramatic shift in technologies, beginning with ocean-going ships and culminating in the Industrial Revolution, saw the most developed belt move north into northern Europe, China, and the Americas. Northern Russia became a superpower while southern India became impoverished and colonized. Diamond argues that such dramatic changes demonstrate that the current distribution of wealth is not due to immutable factors such as climate or race, citing the early emergence of agriculture in ancient Mesopotamia as evidence.
Diamond also notes the feedback effect in the growth of medical technology, stating that more research money goes into curing the ailments of northerners.
Ticks, mosquitos, and rats, which continue to be important disease vectors, benefit from warmer temperatures and higher humidities. There has long been a malarial belt spanning the equatorial portions of the globe; the disease is especially deadly to children under the age of five. Notably it has been almost impossible for most forms of northern livestock to thrive due to the endemic presence of the tsetse fly.
Jared Diamond has linked domestication of animals in Europe and Asia to the development of diseases that enabled these countries to conquer the inhabitants of other continents. The close association of people in Eurasia with their domesticated animals provided a vector for the rapid transmission of diseases. Inhabitants of lands with few domesticated species were never exposed to the same range of diseases, and so, at least on the American continents, succumbed to diseases introduced from Eurasia. These effects were exhaustively discussed in William McNeill's book Plagues and Peoples.
The 2001 Harvard study mentions high infant mortality as another factor; since birth rates usually increase in compensation, women may delay their entry into the workforce to care for their younger children. The education of the surviving children then becomes difficult, perpetuating a cycle of poverty.
In a 2006 paper, John K. Horowitz of the University of Maryland predicted that a 2-degree Fahrenheit temperature increase across all countries would cause a decrease of 2 to 6 percent in world GDP, with a best estimate of around 3.5 percent. United Nations Secretary-General Ban Ki-moon has expressed concern that global warming will exacerbate the existing poverty in Africa.