> I don't know but having lived in all three countries, I don't see that many differences in term of people's character that could explain the difference in murder rate.
No offense, but could this be selection bias? You have certainly only talked to a selected subset of people..
I think if 80% of the U.S. population would want a thorough reform of their justice system, it would happen.
What exactly would an 'external' factor be for you?