A new study reduces the number of people in four personality types: Which one are you?

Data collected from 1.5 million respondents helped researchers reduce their personality into four groups.

Researchers at Northwestern University have found that all four personality types are average, reserved, egocentric, and role models, and that types of people can change over time. Published Monday in the journal Nature Human Behavior, this study could be a useful tool for psychologists or recruiters and challenge existing systems in the field.

"People have been trying to classify personality types since the time of Hippocrates, but the previous scientific literature found this absurd," said William Revelle, a professor of psychology, in a statement. "Now, these data show that there are higher densities of certain types of personality."

Although Revelle co-authored the study, he was initially skeptical. Personality types as a concept are still controversial in psychology because it can be difficult to find scientific evidence of types. Other studies have used small groups, so scientists have struggled to replicate the results. Revelle challenged the other authors to prove that the types existed.

"Personality types only existed in self-help literature and had no place in scientific journals," said Luís Amaral, professor of chemical engineering and technology. in Northwestern, in a statement. "Now we think that will change because of this study."

The responses were collected through various online personality questionnaires developed over the decades. The questionnaires ranged from 44 to 300 questions and the results will be available for other scientists to carry out their own analysis.

The first type of personality is "average". People in the middle group are described as extroverted and strongly neurotic. Women are more likely to be in this type of personality and this is the most common, according to research. The type "reserve" is not very extrovert or neurotic. Instead, they are pleasant and conscientious, as well as emotionally stable. Those of the reserve cluster and the middle cluster are not very open.

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A group of people sitting on a bench in Nice, France. New research has revealed that there are four main groups of personality types: average, reserved, egocentric, and pattern. ERIC GAILLARD / REUTERS

"Egocentric" people are described as very extroverted and are not considered very pleasant, conscientious or open. As people get older, the number of those who fall into the "egocentric" category drops significantly among men and women.

"These are people you do not want to hang out with," said Revelle.

The types of "models" are distinguished by their extroverted, open, pleasant and conscientious character, with the exception of neuroticism. People are more likely to be in the category of models as they get older and women are more likely to be in this group than men.

"These are people who are reliable and open to new ideas," said Amaral. "They are good people to be in charge of things – in fact, life is easier if you have more relationships with models."

Over time, it seemed that people were becoming less neurotic and more pleasant and conscientious.

"When we look at large groups of people, it is clear that there are trends, that some people may change some of these characteristics over time," Amaral said. "This could be the subject of future research."