It is generally thought that science helps good ideas triumph over bad. The weight of evidence eventually pushes false claims aside. But some ideas march onward despite the evidence against them. The discredited link between vaccines and autism continues to cause mischief and climate change skeptics continue to resurrect dead science. Why, then, are some bad ideas so hard to kill? A striking example of such a “zombie theory” comes from personality psychology. Personality psychologists study human individuality – how and why individuals differ in their patterns of behaviour and experience, and how those differences influence our lives. For almost 50 years, an idea with a vexing immunity to evidence has needled this field. This idea is called situationism. Is personality an illusion? Introduced in the 1960s by American psychologist Walter Mischel, situationism is the idea is that human behaviour results only from the situation in which it occurs and not from the personality of the individual. In his 1968 book Personality and Assessment, Mischel claimed that the whole concept of personality is untenable because …