FEAR is devastatingly debilitating. . . if a 300lb black man charged in and started beating some guy standing next to me, I would've done 1 of 2 things:
1. Run like a mo' fo' and forget the pizza.
2. Ask the owner behind the counter for a weapon to impale his fat blackness with, cut his legs up a little so he'll bleed fast and go down, then call the cops.
I'm leaning towards 1 since I don't want a record that could hurt my chances of getting a job, car, house, or renting a place. But that's just me. That's the easiest explanation for the "ByStander Effect".