The 20th century closed with many lamenting civilian killings by the state as its greatest evil. By one estimate, governments killed as many as 170 million civilians from 1900 to 1987 - more than all the soldiers killed in the wars in the 20th century. This new working paper, co-authored by CGD non-resident fellow William Easterly, explores the relationship between the occurrence and cruelty of episodes of mass killing and the levels of development and democracy across countries and over time using a newly assembled dataset spanning from 1820 to 1998. The study finds that massacres are more likely at intermediate levels of income and less likely at very high levels of democracy. The study does not find evidence of a linear relationship between democracy and the probability of mass killings. In the 20th century, discrete improvements in democracy are systematically associated with less cruel massacre episodes. Episodes at the highest levels of democracy and income involve relatively fewer victims.