We tend to behave in a way that supports the impression of a positive and consistent self-image. When things go well in our lives, we attribute it to ourselves; when they go badly, it's the fault of other people, or the situation we were put in – an effect known as the "fundamental attribution error".
We think the same way for groups that we identify with. Psychologists have found this group identification to be a very robust effect, and its power is so great that – like a number of the other effects above – it changes how we see the world. The classic illustration of this effect is sports fans' memories of their team's performance in a match. Fans systematically misremember, and misinterpret, the behaviour of their own team compared with the opponents. A match in which both teams appear equally culpable of committing fouls to an impartial observer will be seen by a partial fan as one characterised by far more fouls by the opposing team than their own.