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It’s hard to believe now, but introversion was once a mostly misunderstood personality trait. Now, it’s the subject of countless books and articles and listicles (and, more recently, parodies of listicles). And as more regular, non-scientist types started to talk about introversion, psychologist Jonathan Cheek began to notice something: The way many introverts defined the trait was different from the way he and most of his academic colleagues did.

“When you survey a person on the street, asking them to define introversion, what comes up as the prototypical characteristics … are things like thoughtful or introspective,” said Cheek, a psychology professor at Wellesley College. And yet neither of these things are part of the definition of the trait according to scientific literature. In the bulk of the research on personality psychology, introversion is usually defined by what it is not: extroversion. If extroverts are assertive and enthusiastic individuals who thrive in highly stimulative social environments, then introverts are the opposite, so the academics’ way of thinking about it goes. What everyday introverts think about it doesn’t really factor in.

This is actually a problem that was identified at least as early as 1980, when one study found that the “scientific” and “common-sense” definitions of introversion didn’t quite match up. And the more Cheek and his colleagues, including graduate students Jennifer Grimes and Courtney Brown, thought about it, and the more self-described introverts they interviewed, the less correct this one-size-fits-all definition seemed. There’s not just one way to be an introvert, Cheek now argues — rather, there are four shades of introversion: social, thinking, anxious, and restrained. And many introverts are a mix of all four types, rather than demonstrating one type over the others. (Scroll to the bottom for a quiz, borrowed from Cheek, to find out your own type.)

Taken together, the first letter of the four types spells out STAR, which is what Cheek named his model. He designed it by surveying about 500 adults, ranging in age from 18 to 70, asking them about things like their preference for solitude, or how inclined they are to daydream. The uniting principle of all four kinds is, of course, a tendency to turn inward rather than outward — but beyond that, it gets more complicated.

Here’s a brief description of each:

Social: Social introversion is the typical definition of introversion, in that a preference for socializing with small groups instead of large ones. Or sometimes, it’s a preference for no group at all — solitude is often preferable for those who score high in social introversion. “They prefer to stay home with a book or a computer, or to stick to small gatherings with close friends, as opposed to attending large parties with many strangers,” Cheek said. But it’s different from shyness, in that there’s no anxiety driving the preference for solitude or small groups.

Thinking: This one is a newer concept. People with high levels of thinking introversion don’t share the aversion to social events people usually associate with introversion. Instead, this style of introversion just means a person who tends to be introspective, thoughtful, and self-reflective. “You’re capable of getting lost in an internal fantasy world,” Cheek said. “But it’s not in a neurotic way, it’s in an imaginative and creative way.” Sounds nice.

Anxious: Unlike social introverts, anxious introverts may seek out solitude because they feel awkward or self-conscious around other people, because they’re not very confident in their own social skills. But, often, their anxiety doesn’t fade when they’re all alone. This kind of introversion is defined by a tendency to ruminate, to turn over and over in their minds the things that might or could or already have gone terribly wrong.

Restrained: You could also just call this reserved. Restrained introverts prefer think before they speak or act. They also might take a while to get going — they can’t, for instance, wake up and immediately spring into action.