The psychologists call it "deindividuation". It's what happens when social norms are withdrawn because identities are concealed. The classic deindividuation experiment concerned American children at Halloween. Trick-or-treaters were invited to take sweets left in the hall of a house on a table on which there was also a sum of money. When children arrived singly, and not wearing masks, only 8% of them stole any of the money. When they were in larger groups, with their identities concealed by fancy dress, that number rose to 80%. The combination of a faceless crowd and personal anonymity provoked individuals into breaking rules that under "normal" circumstances they would not have considered.