The word mobbing is a bullying behaviour that was first recognized in continental Europe. It involves situations where a target is selected and bullied by a group of people rather than by one individual.
Amanda Todd was mobbed by 50 kids and there was never any follow up. How do you hold 50 kids accountable...??
They were obviously part of some kind of "group think" that allowed them to hurt Amanda as a group. "Group Think" is a known psychological phenomenon that allows everyone in a crowd to follow along and possibly do and say things they would never feel safe doing or saying on their own. (An example is a riot situation)
Alternatively, there is another theory that may help us understand this behaviour and be better able to address it...
Every group has a leader. This leader may be an extrovert who is very obviously coercing or encouraging group members into attacking the target. Or the leader may be an introvert, likely operating in the background coercing and manipulating group members into attacking the target. They are much more difficult to identify but with some investigation they can be found.
In a group bullying situation, the leader encourages bystanders to join in or the kids get sucked into a "group think" scenario. These kids may be supporters, cohorts, copycats, immature or emotionally needy individuals who engage in hurting the selected target.
The target if they decide to try and fight back may be deceived into fighting, blaming and trying to hold accountable the bullies of the attacking group as a whole instead of identifying and holding accountable the leader.
The main reason the lead bully gets away with his (or her) behaviour repeatedly is they appear to be so charming and charismatic. Caring adults fail to see the warning signs of being a bully.
I believe when dealing with mobbing the answer is found in both theories.
In a group bullying situation keeping in mind this can happen online as well as offline, we need to identify the lead bully and concentrate on holding this leader accountable. This leader will need help to build compassion and be willing to make it right.
At the same time we need to educate the followers about the power of "group think" and the importance of them having the confidence, compassion and courage not to fall prey to a leader who would hurt others and get those who follow in to trouble.
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