The Greek physician Hippocrates argued that four bodily “humors” shaped human behavior, which, in turn, resulted in four fundamental personality types. Today, people put more faith in Myers-Briggs tests and BuzzFeed quizzes than the formative power of their phlegm, but the fascination with personality types remains. The issue is that scientists remain undecided on whether there are personality types at all. However, in a study released Monday in Nature Human Behaviour, a team of researchers from Northwestern University announced there was some truth to old Hippocrates’ theory.

As it turns out, he was right about the number of personality types that exist among humans. This finding, which shocked the study’s authors, emerged from four large data sets comprising more than 1.5 million participants. Co-author and psychology professor William Revelle, Ph.D., says that before this study he was convinced there were no personality types at all. Now he’s convinced that personalities fall into four distinct clusters that still aren’t completely separable: types the team refers to as average, “role model,” reserved, and self-centered.

Study co-author and postdoctoral fellow Martin Gerlach, Ph.D., tells Inverse that “in addition to our careful computational analysis, the strongest argument [for personality types] is that we can find the same types across four different datasets that use different questions and were collected by different researchers.”

Using a standard clustering algorithm, the team searched these massive data sets and, on a quadrant graph, plotted out how the individuals in the four data sets manifested five widely accepted basic personality traits: neuroticism, extraversion, openness, agreeableness, and conscientiousness. While previous studies raised questions about the validity of personality types, researchers generally agree that these traits, known as the Big Five, are reliable and replicable domains of human personality.

Plotting the ways that people manifested combinations of the traits caused four major types to emerge.

Average

average personality
The traits paired with "average."

The “average” type is called average for a reason. Study co-author and professor of chemical and biological engineering Luís Amaral, Ph.D., says in an accompanying video that “there is very little to say about average,” and so it’s not surprising that most people fit into this cluster. While still scoring a bit higher in terms of neuroticism and extroversion than in agreeableness and conscientiousness, average people, fittingly, are characterized by an average score in all traits. There are slightly more women who are “average” than there are men.

“Looking across all datasets, the average type is characterized by very neutral scores — close to zero — on all five traits,” explains Gerlach. “Most individuals’ traits are centered around this region in space. Therefore, a typical respondent would fall close to this type.”

Role Model

role model
Traits for a "role model."

In this study, people with the “role model” personality are described as nice, agreeable, and open-minded people. They score low in neuroticism and high in all other traits. As the name suggests, a “role model” is someone you’d like as a role model: dependable and interested in new ideas. Women are more likely to be role models than men, and respondents older than 40 were “strongly overrepresented” in this group. The good news for young folks is that personality types are not fixed. As people age, the authors write, they tend to become less neurotic and more conscientious, effectively shifting their personality type.

Self-Centered

self-centered
Descriptive traits for the "self-centered."

Teen boys, the study authors write, are very over-represented in the self-centered groups, whereas women under the age of 15 are “vastly underrepresented.” Women over the age of 60 also didn’t really register as self-centered, with this group “showing a more than five-fold decrease in appearing in the vicinity of this cluster.” The folks that are self-centered are described as not hard-working, disagreeable, not open-minded, and extraverts.

“These are the people you don’t want to hang out with,” says Revelle.

Reserved

personality traits
Traits for the "reserved."

Those who are “reserved” are not very neurotic but aren’t very open, either. They are, at least, emotionally stable, so kudos for that. While the other traits appeared to cluster somewhat around certain ages or genders, reserved is a neutral trait that can apply to any demographic. Agreeable and conscientious, a reserved individual is probably a good person to have around.