Why you're content to silently curse out line-cutters in your head

Milgram's most interesting insights are his attempts to explain why people don't intervene. Are people just too scared? Not necessarily:

Group formation is difficult when people are stood one behind the other, all facing in the same direction. Consequently social order is weak.

1. Challenging queue-jumpers could mean losing your own place in the line.
2. Social systems have to tolerate some deviance otherwise they may quickly break down, i.e. a fight may start and everyone is delayed while it is sorted out.
3. The line is co-opting those who threaten it by tacitly accepting them so that they gain an interest in the queue and the queue becomes stronger.