Since the Republican primaries of 2015-16, some people have turned to psychiatry in an effort to locate the irrational wellsprings of Trump’s victory, but so far little progress has been made. This is because most of the effort has gone into analysing Trump, who is often described as suffering from ‘narcissistic personality disorder’. Not only are such diagnoses, made from a distance, implausible; they also fail to address a more important question: the nature of Trump’s appeal. Constituting something close to a third of the electorate, his followers form an intensely loyal and, psychologically, tight-knit band. They are impervious to liberal or progressive criticisms of Trump or his policies. On the contrary, their loyalty thrives on anti-Trump arguments, and digs in deeper. There is an older body of psychological thought, however, that illuminates the kind of tight bond Trump has forged with a significant minority of Americans.