Researchers led by Northwestern Engineering’s Luis Amaral sifted through data from more than 1.5 million questionnaire respondents to find at least four distinct clusters of personality types exist — average, reserved, self-centered, and role model — challenging existing paradigms in psychology.

“People have tried to classify personality types since Hippocrates’s time, but previous scientific literature has found that to be nonsense,”said co-author William Revelle, professor of psychology at Northwestern University’s Weinberg College of Arts and Sciences. 

“Now, these data show there are higher densities of certain personality types,” said Revelle, who specializes in personality measurement, theory, and research.

The new study appears in Nature Human Behaviour. The findings potentially could be of interest to hiring managers and mental healthcare providers.

Initially, Revelle was skeptical of the study’s premise. The concept of personality types remains controversial in psychology, with hard scientific proof difficult to find. Previous attempts based on small research groups created results that often were not replicable.

“Personality types only existed in self-help literature and did not have a place in scientific journals,” said Amaral, Erastus Otis Haven Professor of Chemical and Biological Engineering at the McCormick School of Engineering. “Now, we think this will change because of this study.”

The new research combined an alternative computational approach with data from four questionnaires, attracting more than 1.5 million respondents from around the world. The questionnaires, developed by the research community over the decades, have between 44 and 300 questions. People voluntarily take the online quizzes, attracted by the opportunity to receive feedback about their own personality.

These data are now being made available to other researchers for independent analyses.

“A study with a dataset this large would not have been possible before the web,” Amaral said. “Previously, researchers would recruit undergrads on campus and maybe get a few hundred people. Now, we have all these online resources available, and data is being shared.”

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“Machine learning and data science are promising but can be seen as a little bit of a religion,” Amaral said. “You still need to test your results. We developed a new method to guide people to solve the clustering problem to test the findings.” 

Their algorithm first revealed about 16 personality clusters using traditional clustering methods. The researchers then imposed additional constraints, winnowing down the clusters to four distinct personality types. 

“The data came back, and they kept coming up with the same four clusters at higher densities than you’d expect by chance, and you can show by replication that this is statistically unlikely." Revelle said. "The methodology is the main part of the paper’s contribution to science." 

To ensure the new clusters of types were accurate, the researchers used a notoriously self-centered group — teenaged boys — to validate their information.

“We know teen boys behave in self-centered ways,” Amaral said. “If the data were correct and sifted for demographics, they would they turn out to be the biggest cluster of people.”

Indeed, young males are overrepresented in the Self-Centered group, while females over 15 years old are vastly underrepresented.

Along with serving as a tool that can help mental health service providers assess for personality types with extreme traits, Amaral said the study’s results could be helpful for hiring managers looking to ensure a potential candidate is a good fit or for people who are dating looking for an appropriate partner.

And good news for parents of teenagers everywhere: as people mature, their personality types often shift. For instance, older people tend to be less neurotic yet more conscientious and agreeable than those under 20 years old. 

“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," Amaral said. "This could be a subject of future research."

The research was funded by a gift from Mac and Leslie McQuown along with support from Department of Defense Army Research Office, and the National Science Foundation.

Julianne Hill, director of strategic communications at the McCormick School of Engineering, is the author of this story.