Personality tests are beloved by high school guidance counselors and self-help book authors - but less so by many scientists.
"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, Northwestern professor of psychology, in a statement.
"Now, these data show there are higher densities of certain personality types". His latest research published Monday in Nature Human Behavior points to the opposite conclusion.
Initially, Revelle was skeptical of the study's premise.
Scientists have discovered four main personality types, but put the brakes on your visions of a Harry Potter-style Sorting Hat.
Previous attempts based on small research groups - whereas the new study used an online personality quiz with between 44 and 300 questions.
Luis Amaral, professor of chemical and biological engineering at Northwestern and leader of the study, said that older people and women were the most likely fall into the category. Personality isn't fixed, but changes as we grow and evolve.
To define those categories, the researchers studied data from four online questionnaires that attracted more than 1.5 million respondents from around the world. They are now making the information available to other researchers, so they can do independent analyses.
"A study with a dataset this large would not have been possible before the web", Amaral said. "I would expect that the typical person would be in this cluster", said Martin Gerlach, a postdoctoral fellow in Amaral's lab and the paper's first author. Using machine-learning algorithms, they scanned the first data set of almost 150,000 responses looking for clusters of people who scored similarly on the five traits.
Reserved The Reserved type is emotionally stable, but not open or neurotic.
Meanwhile, people in the "self-centered" cluster, the researchers found, are fairly disagreeable, not very conscientious and not very open to new experiences. Likelihood of being a role model increases dramatically with age.
Traits include dependability, openness and thinking about situations logically. "These are good people to be in charge of things". Some people may fit one perfectly, while others are more loosely associated with one of the camps. More women than men are likely to be role models.
Questionnaires filled out by the group assessed five well-established traits: openness, agreeableness, conscientiousness, neuroticism (level of emotional stability), and extraversion (quality of being outgoing). While people live across the country, it's easy to spot high-density areas like New York City, Los Angeles and Chicago, which are each home to far more people than, say, Cleveland or Tallahassee. "But whether those clusters, the four clusters they found, reflect some true underlying reality about people is something that requires other forms of evidence". At first, the algorithm returned downright unbelievable results, including up to 20 different personality clusters.
"At first, they came to me with 16 personality types, and there's enough literature that I'm aware of that says that's ridiculous", Revelle said. "I believed there were no types at all". "You still need to test your results. We developed a new method to guide people to solve the clustering problem to test the findings". "More useful than saying what city you live in".
After refining the calculations, the team ended up with the four clusters reported in their new study.
Indeed, young males are overrepresented in the Self-Centered group, while females over 15 years old are vastly underrepresented.
The researchers said their results could be of potential use to mental health workers trying to diagnose extreme personality traits and also hiring managers looking for a particular character in applicants for a post.
The researchers note that some people will display traits relevant to more than one personality type, and also that a personality is not permanent; it can shift and alter over time, depending on age, lifestyle circumstances and other extraneous factors.