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. He has a chance to influence it. But its decision is final.

The Sorting Hat is fantasy. But its impetus to divide people into personality types is real.

The ancient Greek physicianA medical doctor. Hippocrates is believed to have theorised that the balance of fluids in a person's body determined the type of person we were.¹ Today, many believe in horoscopes. These decide your character based on the date on which you were born.² And the Harry Potter house system has become popular online.

But we do not just sort each other for fun. Personality tests are big business. Companies use them to decide who to hire and who to put into teams. And individuals use them to learn about themselves, to find romantic pairings and even seek spiritual enlightenment.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" based on how they take decisions, take in information and live their lives.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 and respond to them in a way that allows for smooth social interactions."

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. Its results do not sync well with job performance. 

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.

Some even believe that the tests are harmful. Academic Merve Emre claims that employers use tests to make people "bind themselves to their work freely and gladly" by convincing them that the job matches their personality. 

Worse, according to disability advocate Lydia XZ Brown: "Personality tests are by and large constructed to be ableist, to be racist, to be sexist, and to be classist". A company could use one to filter out groups that they do not want to employ.

Others think tests hold at least a grain of truth. Categories are one of the main ways we understand the world. And tests bring together collective insight into what categories do exist. They may not fully capture someone, but they can teach us things we otherwise would not know.

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

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 alter ourselves or make our test results a guide for how we behave, perhaps we will make them retroactively come true.