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 sceptics 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 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 people behave differently in different situations.

If there are no consistent patterns in our behaviour and we merely react, chameleon-like, to different contexts, then our sense of an enduring personality is illusory. With that bombshell, the person-situation debate erupted.

Situations versus personality

The notion that situations influence behaviour is patently true. Could we even imagine a world in which people did not adjust their behaviour to different contexts – from job interviews to romantic dinners?

Personality psychologists have shown time and again that the demands of situations shape and guide our behaviour. As one of the founders of personality psychology, Gordon Allport, observed in the 1930s:

We all know that individuals may be courteous, kind and generous in company or in business relations, and at the same time be rude, cruel and selfish at home.

But does this flexibility mean there is no consistency in behaviour, rendering the whole notion of personality untenable? Is there no tendency in some individuals to be consistently more courteous than others?

Here the empirical record disagrees. There is significant consistency of behavioural differences between people, both over time and across situations. These tendencies are well captured by measures of personality, as study after study has shown. This tells us that stable differences in personality are real and observable – they are not illusions.

As for the importance of personality, the evidence shows that personality traits are reliable predictors of many important life outcomes, from social behaviour to job performance, from educational achievement to health and well-being.

A case of consistency: the marshmallow study

Ironically, a particularly famous example of the stability and power of personality came from Mischel’s own research, which, as one report points out, drives him crazy.

In the marshmallow study, Mischel measured young children’s willpower by timing how long they could resist the temptation of a delicious treat. This simple test, it turns out, is a measure of the personality trait called conscientiousness. It also predicts the same outcomes later in life that conscientiousness does, including higher educational achievement and lower drug use. The facts that have emerged from this research are simply incompatible with situationism.

Laying situationism to rest

Even before it was disproven by the evidence, Mischel’s theory of situationism contained a logical non sequitur. Specifically, it assumed that a person’s behaviour can only be 100% consistent or else inconsistent – in which case there is no such thing as personality.

But why should the observation of changeable behaviour imply the absence of personality? By this reasoning, we should dismiss the whole notion of climate because weather is changeable.

By the 1990s, most personality psychologists considered situationism a dead duck. A prominent review of the literature concluded that the debate had, at last, fizzled out. The field was moving on and looking forward.

But the theory didn’t die.

Back from the dead

Time and again, the spectre of situationism has reappeared, causing a groaning sense of déjà vu for personality psychologists.

The theory has even spread beyond psychology, with a prominent behavioural economist recently claiming that Mischel’s “great contribution to psychology” was to show that there is “no such thing as a stable personality trait”.

Despite being buried by decades of research, situationism keeps kicking. According to one commentator, it “has morphed into something beyond the veracity of its arguments”. It has become an ideology.

In June this year, Mischel wheeled out situationism once again, this time on an episode of the NPR Invisibilia podcast titled The Personality Myth. Once again, we’re told “ultimately it’s the situation, not the person, that determines things.”

This baseless message drew sharp criticism on social media by several eminent personality psychologists.

As one observed:

[…] the contemporary research literature showing that personality traits exist, tend to be stable over time, and influence important life outcomes is never mentioned.

What gives life to bad ideas?

Why is situationism still being revived after decades of refutation? We suspect this can be explained by at least two factors.

The first is our all-too-human preference for lazy thinking. As Daniel Kahneman explains in Thinking Fast and Slow:

When faced with a difficult question, we often answer an easier one instead, usually without noticing the substitution.

In this case, the tricky question, “can our patterns of behaviour be generally stable yet highly changeable?”, is switched for a no-brainer, “is our behaviour perfectly consistent, or not?”

The second explanation may lie in the appeal of a surprising story. Some of the most alluring ideas in science – and to scientists – are those we find unexpected or counter-intuitive. And what could be more counter-intuitive than the thought that there may be nothing at all that makes you you?

The situationist idea that personality is an illusion is an arresting one, but it is false.