The Economist has analysed all international and civil wars since 1900, along with the belligerents’ wealth and degree of democratisation (assigning colonies to their own category). We counted all conflicts involving national armies in which at least 100 people per year were killed, excluding deaths from terrorism, massacres of civilians outside combat, starvation or disease.

The data show a strong correlation between democracy and peace, with a few exceptions. (The United States has been quite bellicose, and its advanced democracy did not prevent a civil war in 1861 that claimed more American lives than any conflict since.) Moreover, the relationship does not seem to be linear. The countries most prone to wars appear to be neither autocracies nor full democracies, but rather countries in between. A similar finding applies to prosperity. Middle-income countries are more warlike than very poor or rich ones.