On Killing by LtCol Dave Grossman, 2009, Excerpts
Psychologists have long understood that a diffusion of responsibility can be caused by the anonymity created in a crowd. It has been demonstrated in literally dozens of studies that bystanders will be less likely to interfere in a situation in direct relationship to the numbers who are witnessing the circumstance. Thus, in large crowds, horrendous crimes can occur by the likelihood of a bystander interfering is very low. In general, the more members in the group, the more psychologically bonded the group, and the more the group is in close proximity, the more powerful the enabling can be.