What effect does mob violence have on the emotional stability of the city?

Mob violence can arise from contagious hysteria, a fairly well documented psychological issue where people feel better to join in rather than resisting large group aggression. As groups become more out of control, the effect is contagious, and the hysteria becomes violent. The effect is like a Powder Keg and eventually dissipates as the mob runs out of steam or is overpowered/controlled by an authority or larger group.