Steven Pinker's Better Angels of Our Nature goes over all of the probably reasons for this in its general survey of the correlation between violence and civiliz

Steven Pinker's Better Angels of Our Nature goes over all of the probably reasons for this in its general survey of the correlation between violence and civilization. And he's mentioned for the Leviathan theory, but that is only one of the areas he touches on. It's the most credible explanatory frameworks for violence that I've come across.