Sitting in the sun on a holiday weekend is an opportunity to reflect on one of the oddest questions in economic geography.  Why are hot countries poor?  If you want to live both in a prosperous country and in the tropics, your choices are more or less limited to Singapore and the far north of Australia.

Perhaps sitting in the sun suggests an immediate answer.  If your home is in Manchester or Dusseldorf, there are fewer distractions from work.  You are not tempted to laze around in a village square with friends and drinks, nor are you  frustrated by the debilitating heat and humidity of many tropical regions. Yet it is not so cold that much of your time, energy and income is devoted to keeping warm. The work ethic is easiest to maintain in drizzle and a light breeze.

Attempting to escape this moralising argument, Jeffery Sachs has emphasised the role of disease.  Hot, humid climates may not be congenial environments for humans, but they are congenial to many unpleasant parasites.  Malaria alone is a major obstacle to economic development.  Another geographic explanation comes from Jared Diamond.  The crops and animals on which modern agriculture is based – such as corn, maize, pigs and sheep – thrive in temperate zones .  In an ingenious twist on this argument, Diamond goes on to argue that this is why Eurasia – a horizontally positioned land mass where agricultural practices could be transmitted to similar climates to the east and west – developed more readily than America, a vertically positioned landmass in which agricultural practice could not easily be transmitted from north to south.

This  European good fortune leads to a different explanation which  relies on the influence of European colonialism.  Acemoglu, Johnson and Robinson (‘The colonial origins of economic development’, AER 2001) point out that you would have observed a very different  relationship between temperature and per capita income if you had looked at the world in 1500, when the Mogul and Aztec civilizations were at their height.   Western Europe developed the political, social and economic institutions that make modern economic prosperity possible. And transmitted these round the world.

But the effects of transmission through settlement were very different from the effects of transmission through colonisation.  If you are going to change the place you live, the most congenial location is  a place slightly (but not too much) warmer than the one you are accustomed to.  (That is why I am writing this column in the South of France.)  And why emigrants of European origin settled in south east Australia, New Zealand, South Africa and the United States, taking their institutions with them.  

      But in India and the Congo Europeans had no interest in permanent settlement:  the institutions they installed there were essentially those of exploitation,  and a legacy of exploitation continued to afflict these countries even after the colonisers returned home.  It is not an accident that Montesquieu was the first person (in 1748) to observe the correlation between climate and prosperity:  not only was he from a colonial power, but he was writing at precisely the time that imperialism was creating the phenomenon he described.

What becomes clear is that all these factors are interdependent.  Malaria is no longer a problem in Italy and the southern United States, but that is not because Italy and the southern United States no longer have climates susceptible to malaria:  it is because these countries have the per capita incomes and public institutions needed to eliminate the problem.  Modern agriculture is most successful in temperate zones :  but that is because the crops and plants that have been most effectively bred are those adapted to the climates with the most advanced agricultural technology.   Singapore could not be a prosperous state without air conditioning:  but it is because it is a prosperous state that it has air conditioning.

And it is these interdependencies which  make the problem of global poverty so intractable.  Prosperity is associated with honest, stable, institutions, the effective use of modern technology, a functioning transport and communications infrastructure, satisfactory levels of public health and hygiene, and many other things:  and to establish any one of these features  without the others achieves very little.  That is why attempts to kick-start economic development have mostly failed:  countries must find their own paths to prosperity.  Today there are some hopeful signs that more states are achieving precisely that.