Augustus
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It seems that many people want to lash out at others but generally hesitate to do so because of fearing immediate and possibly violent retribution.
It seems to me patently incorrect that the main reason people don't behave more like that in real life is simply that they fear a violent response. Otherwise people would do it more often to those they don't fear.
Some people are vindictive and cruel, but I'd say this is a small minority.
People behave very differently in football crowds than they would in direct personal communication. Reserved, polite people will shout and swear aggressively, dance and sing and feel a form of hatred for the opposition. They don't secretly want to do this all the time except that they fear a punch in the face. It's not like they do this to folk IRL who are weaker than them and no threat.
In a local rivalry, the opposition crowd they are doing this towards often contains many of their friends who they will meet for a friendly drink after the match. Online discussion often doesn't have the clear end point after which folk just go back to normal, and increasing polarisation means fewer people meet up with their 'enemies' and return to seeing them as their friends and colleagues.
It's like the ancient and medieval carnival times where normal rules didn't apply and people could let off steam. Behaviour and its rules of etiquette are contextually contingent
People's behaviour is mediated by both positive and negative feedback. Most bad behaviour online is just people seeking status in an arena with different rewards, or people playing a kind of game. As folk are generally isolated from seeing any negative effects of their behaviour, different dynamics apply.
IRL, people are far more likely to refrain from behaviour out of politeness, or for fear of hurting or upsetting someone than they are because they fear a punch in the face.