By using data sets involving more than a million people around the world, researchers at Northwestern University, Illinois, have revealed what they believe were the four major human personality types. The findings have the potential to be used by hiring managers and mental health care providers.

The study titled "A robust data-driven approach identifies four personality types across four large data sets" was published in the journal Nature Human Behaviour on Sept. 17.

Whether in the form of horoscopes or internet quizzes, a lot of us actually take the effort to find out what kind of box we fit into. Among common subjects of fascination, determining your own personality type is a major one, with the Myers–Briggs Type Indicator being a popular tool to do so. But scientists have not been sure whether these tests and categorizations are completely accurate. 

"People have tried to classify personality types since Hippocrates' time, but previous scientific literature has found that to be nonsense," said co-author William Revelle, professor of psychology in the Weinberg College of Arts and Sciences. 

But despite his initial skepticism with the new study, Revelle believes the new findings strongly indicate higher densities of certain personality types in the human population. As part of the study, 1.5 million people from around the world were recruited by the researchers to answer a series of questions and have their results run through an algorithm.

This algorithm helped plot the overwhelming amount of data and identify four broad personality types — average, reserved, role models, and self-centered. 

Before exploring what each type represents, it was important to know the basic traits they were rated and based on. These traits, known as the Big Five, are neuroticism, extraversion, openness, agreeableness, and conscientiousness.

First, "average" was found to be the most common personality type, as suggested by the name. 

"I would expect that the typical person would be in this cluster," said first author of the study Martin Gerlach. People with this type score high in neuroticism and extraversion but score low in openness.

The "role models," on the other hand, scored low in neuroticism and high in all the other traits. Someone like Captain America would probably fall into this category, characterized by being dependable and having the qualities of a good leader.

Next, the "reserved" category comprises of people who are emotionally stable but score low on openness and neuroticism. While they are somewhat agreeable and conscientious, they are not too extroverted. Interestingly, this category saw the most variety in terms of age and gender.

Lastly, the "self-centered" types score particularly high in extraversion and below average in traits like openness, agreeableness, and conscientiousness. "These are people you don't want to hang out with," Revelle said. Teenage boys tended to be overrepresented in this group.

The researchers did emphasize personality types can change over time, which is particularly noticeable when examining how a person changes between their teens and their 20s.

"When we look at large groups of people, it's clear there are trends, that some people may be changing some of these characteristics over time," said Luís Amaral who led the study. "This could be a subject of future research."