Whether they are proposing to build a wall or to exit an international coalition, populist politicians like to pitch themselves as keeping ‘outsiders’ at bay, and it clearly strikes a chord with their home crowd. To understand this phenomenon, evolutionary and social psychologists have offered a simple explanation. Humans, we’re told, have a deep-rooted inclination to mistrust ‘the other’ – people who do not belong to our community or ingroup. Classic work published in 1970 by the Polish-born psychologist Henri Tajfel showed how rapidly and arbitrarily teenage schoolboys form a sense of loyalty to their own group, and a bias…

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