Typecast

"Hmm. Difficult. Very difficult. Plenty of courage, I see. Not a bad mind either. There's talent, oh my goodness, yes - and a nice thirst to prove yourself, now that's interesting... So where shall I put you?"

It is the young Harry Potter's first day at HogwartsThe school for witches and wizards attended by Harry Potter in the fictional series by JK Rowling. . He puts on the Sorting Hat: a talking headpiece that sorts pupils into houses. The hat reaches into Harry's mind and analyses his personality. Its decision is final.

The Sorting Hat is fantasy. But it echoes a real world love for sorting people by personality. Medieval doctors thought that the balance of four fluids in our bodies affected how we behave.¹ Many people believe in horoscopesA forecast of someone's future based upon the positions of the stars and planets at the time of their birth. , which decide your personality based on your star sign.² And the Harry Potter house system itself has become popular online.

Personality tests are big business. Companies use them to decide who to hire and who to put into teams. And people use them to learn about themselves, to find romantic pairings and even seek spiritual enlightenmentAn intellectual movement of the eighteenth Century that ruthlessly criticised religion, traditions and inherited privilege..³

The best-known is the Myers-Briggs Type Indicator (MBTI). Over two million people take it every year. Invented in 1944 by a mother-daughter team, this sorts people into 16 "types".⁴

Supporters think tests give us an idea of our strengths and weaknesses, or what we need to work on to be happier. "In theory," writes psychologist Jennifer Fayard, "knowing someone's MBTI type allows us to quickly and easily understand them."

But the MBTI has many critics. Neither Myers nor Briggs had any training in psychology or testing. Over half the people who take it a second time get a different score.

Most personality tests are self-reported. And we are experts at deceiving ourselves. If someone taking a Hogwarts house test does not want to be sorted into SlytherinOne of the four houses at Hogwarts, the fictional school attended by Harry Potter. Slytherin has a reputation for producing evil witches and wizards., for instance, they can choose to answer in a way that prevents that outcome.

Are personality tests true? 

Yes: They might not always offer a perfect match for everyone. But personality tests are accurate enough to teach us about ourselves and help us navigate life.

No: Personality tests are the latest in a series of systems designed to make people easier to understand. But the truth is that people are too complex and varied to be neatly sorted into boxes.

Or... Researchers have noticed people altering their behaviour to match their test results. If we make our test results a guide for how we behave, perhaps we will make them come true.