“This planet has – or rather had – a problem, which was this: most of the people living on it were unhappy for pretty much of the time. Many solutions were suggested for this problem, but most of these were largely concerned with the movements of small green pieces of paper, which is odd because on the whole it wasn’t the small green pieces of paper that were unhappy.” – Douglas Adams

In this pithy paragraph, the sorely missed Douglas Adams sums up a puzzling paradox of modern life – we often link happiness to money and the spending of it, even though both proverbs and psychological surveys suggest that the two are unrelated.

Across and within countries, income has an