I wonder why they excluded robbers and thugs from their results. It seems as though that is an important demographic to consider when looking at anything involving weapons and violence.
And yes, jobs are vital! I'm sure there is a strong correlation between a person's perception of their self-worth and their ability to provide for themselves and their families. Also, many jobs that earn a low wage are quite labor intense - so perhaps people also lose the energy and time to cause a ruckus after working all day. :-)