Why Do Americans Have Such A High Homicide Rate?

Steven Pinker's theory:

My own guess is that Americans (particularly in the south and west) never really signed on to a social contract that gave government a monopoly on the legitimate use of violence, as Europe did. Americans not only retain the right to bear arms but believe it is their responsibility, not the government’s, to deter harm-doers. With private citizens, flush with self-serving biases, acting as judge, jury, and executioner, body counts can pile up as trigger-happy vigilantes mete out rough justice. This may be a legacy of the long periods of anarchy in the mountainous south and frontier west, and of the historical failure of the police and courts to serve African American communities.

Earlier coverage of Pinker's theory of growing pacificism here, here and here. The Economist charts the global homicide rate, noting "most of the world’s 468,000 intentional homicides in 2010 were in Africa (36%) and the Americas (31%), where the murder rate was 15-16 per 100,000 people, more than twice the global average of 6.9 per 100,000."