Like the popularity of the Myers-Briggs personality test, the intro-extro divide speaks to people`s inborn need for neat explanations for why they are who they are -- and less, one might argue, to the scientific research itself.

Like so much in life, it would help if we defined terms. "Quiet" author Susan Cain says she likes to define introverts as "people who prefer quieter, more minimally stimulating environments." As Cain has written, introversion and shyness are often conflated, which creates one of the first problems.

As Colgate University psychologist Rebecca Shiner told Science of Us, shyness is a hard trait to form a solid concept around, since there are a bunch of different reasons that people might not want to be sociable.

To get a clearer picture of this, it helps to examine people`s early lives, to look at how personality traits manifest as what psychologists called "temperament" -- or the raw ingredients of personality. Shiner, who is also co-editor of the "Handbook of Temperament," says that personality traits are the fuller expression of temperamental traits, as they have more room to manifest as infant turns to child to adult.

Apparently there are four kinds of introversion

The roots of what are popularly taken to be introversion and extroversion show up in infancy: positive emotionality, or extroversion, and negative emotionality, or neuroticism. In an email, Shiner explained that positive emotionality, as the name suggests, includes the tendencies to experience positive emotions, to engage in lots of social interactions, to be super energetic, and to be eager about getting into rewarding situations.

As you get older, positive emotionality also expresses itself as assertiveness -- people listen to the loudest person in the room.

Negative emotionality disposes you to a range of uncomfortable emotions, from anxiety to irritability to insecurity to vulnerability. Basically, it makes you really good at identifying threats in the environment. (As one researcher told Science Of Us, anxiety is "largely responsible for the survival of the species," since detecting threats helps keeps humans alive.)

People who rate highly on negative emotionality -- neurotics, that is -- are sensitive to threats, making them more easily distracted at work and harder to stay happily married to.

Fixating on introversion limits your life

If we may wade into useful, technical jargon, the important thing to recognize, Shiner says, is that positive and negative emotionality are "orthogonal," meaning how you stand on one is unrelated to the other. "You could have people who are highly extroverted and highly neurotic," she says.

"Those are people who experience a lot of emotions, they`re more emotionally intense, may have approach-avoidance conflict, they`re attracted to awards and worried about threats. You could have people low on extroversion and low on neuroticism, and they tend to be less emotionally intense. I think of the stereotypical engineer -- not worried about a lot of things, and not as sociable."

These are biological systems that everybody has, but in varying degrees. Extroversion is the biological system for your sensitivity to rewards; neuroticism is your sensitivity to threats. This is part of why shyness is such a muddy concept: not wanting to hang out with people as much as others could stem from not being attracted to the rewards of socializing or from feeling anxious about the social risks involved. To Shiner, humanity wouldn`t have gone very far without both of those systems: You`ve got go after rewards and avoid dangers if you`re going to thrive.

Understanding the two types of extroversion

At the same time, traits aren`t monolithic: There`s evidence that big life events change personality traits, and that dispositional mindfulness -- like being able to describe your experience -- can protect against the most negative effects of neuroticism, like depression. Similarly, a huge portion of who you are are your goals: If you`re normally super attracted to social rewards, you`ll sculpt the space for solitude if you`re cramming for finals, applying to law school, or writing a book proposal.

To see your identity in any single trait -- introvert, extrovert, or neurotic -- is to limit your life.



/CNN/

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