Are you an introvert? It depends on which book you read.
Here's a sampling of the various conceptualizations of introversion in pop culture :
- Preference for quiet, minimally stimulating environments: Quiet by Susan Cain
- Preference for concentration and solitude: The Introvert's Way by Sophia Dembling
- Rechargeable battery: The Introvert Advantage by Marti Olsen Laney
- Thoughtful-introspective: Solitude by A. Storr
- Shy-socially anxious: The Gift of Shyness by A. Avila
- Artistic-sensitive-creative: The Highly Sensitive Person by E. Aron
- Literary-observer: Jane Austen, The Complete Novels
- Worried: The Positive Power of Negative Thinking by J. Norem
- Lonely-isolated: Just Your Type by P. Tieger
- Loner-alone by preference: Party of One by A. Rufus
- Low Energy: High Energy Living by R. Cooper
Historically, there has been just as much confusion in the psychological literature. Carl Jung originally defined introversion as a focus on one's "inwardly directed psychic energy". However, in the 30s, the psychologist J.P. Guilford showed that various attempts to measure Jung's conceptualization of introversion resulted in multiple, distinct factors. In other words, there didn't appear to be a single dimension of personality that captured all of introversion.
In the 60s Patricia Carrigan echoed this point, arguing that introversion was not effectively captured by a single scale. She cautioned that if the phrase introversion is to continue to be used, "care must be taken to specify its conceptual and operational referent. What appear to be minor distinctions between the various conceptions may in fact be crucial ones." In the 70s, the heated debate continued, with a much older Guilford arguing with H.J. Eysenck over whether introversion can possibly, or even should, be captured by a single scale.
All seemed to be settled in the 90s with the emergence of the "Big Five" framework of personality. The five main factors of personality-- extraversion, neuroticism, agreeableness, conscientiousness, and intellect/imagination-- were empirically derived based on what patterns of behavior tend to go together within individuals. Under this framework, introversion is merely defined as the opposite of extraversion.
In the Big Five framework, extraversion comprises two main aspects: enthusiasm (reflecting sociability, positive emotions, and warmth) and assertiveness (reflecting the tendency to take charge, become a leader, and captivate attention). The common factor seems to be high sensitivity to rewards in the environment-- which due to the highly social nature of humans throughout the course of human evolution, most prominently consists of rewards associated with social attention.
Therefore, under the dominant personality framework in modern psychology, if you score low in enthusiasm and assertiveness, you're an introvert . Sorted.
Not so fast.
An Alternative Approach to Introversion
Here's the problem: the Big Five framework forces a definition of introversion onto people, many of whom do not conceptualize introversion in the same way. For instance, one study did a comparison of common-sense/everyday notions of introversion and 'scientific' conceptions of introversion. They found that the most prototypical characteristic of introversion, as identified by the general public, was the following item:
"Thinks often about himself/herself"
Clearly, many people equate introversion with introspection. In the Big Five framework, however, this item is classified as part of the intellect/imagination domain of personality, not the introversion domain. So there's a serious mismatch between folk definitions of introversion and scientific definitions. People who view themselves as introverted because they are highly introspective are being told by scientists: "You aren't really introverted based on patterns of covariation among the general population." To which everyday people rightfully respond: "WTF?" 
As noted personality psychologist Jonathan Cheek told me, "by invalidating the ordinary language meaning of introversion by defining it solely as the opposite of Big Five Extraversion, the Big Five researchers are guilty of Psychological Imperialism ." Influenced by the seminal work of Jung, Guilford, and Carrigan, Cheek and his colleagues have decided to take a different approach, by focusing on the phenomenon of introversion on its own, free from having to be force-fit into one scheme or another.
As Carl Jung said, each individual is ultimately a unique crystal, but type theories can be helpful for navigating social life. Embracing this Jungian philosophy, Cheek and his colleagues argue that when people use the term "introversion", they should never just use it by itself. Instead, they argue that researchers should put a specific modifier in front of the term. What modifiers could be used?
In her masters thesis (written under the advisement of Cheek), Jennifer Odessa Grimes defined four meanings of introversion: social, thinking, anxious, and restrained (which happens to form the positive acronym STAR). It's possible to score high or low on either of these flavors of introversion. For instance, you could be low in social introversion by preference but not be particularly anxious in the presence of people. Or you could suffer from crippling social anxiety, but still have the desire to be highly social. Or any other combination of these four meanings of introversion.
By this point you're probably wondering what kind of introvert you are. Well, you're in luck. There's a new test for that.
What Kind of Introvert Are You?
Putting together a large number of contemporary tests of personality, Grimes, Cheek, Julie Norem, and Courtney Brown created the STAR test to measure four kinds of introversion.  To figure out your primary introverted type, take this online test:
To find out where you stand on each of the four meanings of introversion, answer the following questions by deciding to what extent each item is characteristic of your feelings and behavior. Fill in the blank next to each item by choosing a number from the following scale:
1 = very uncharacteristic or untrue, strongly disagree
2 = uncharacteristic
3 = neutral
4 = characteristic
5 = very characteristic or true, strongly agree
____ 1. I like to share special occasions with just one person or a few close friends, rather than have big celebrations.
____ 2. I think it would be satisfying if I could have very close friendships with many people.
____ 3. I try to structure my day so that I always have some time to myself.
____ 4. I like to vacation in places where there are a lot of people around and a lot of activities going on.
____ 5. After spending a few hours surrounded by a lot of people, I am usually eager to get away by myself.
____ 6. I do not have a strong need to be around other people.
____ 7. Just being around others and finding out about them is one of the most interesting things I can think of doing.
____ 8. I usually prefer to do things alone.
____ 9. Other people tend to misunderstand me—forming a mistaken impression of what kind of person I am because I don’t say much about myself.
____ 10. I feel drained after social situations, even when I enjoyed myself.
____ 1. I enjoy analyzing my own thoughts and ideas about myself.
____ 2. I have a rich, complex inner life.
____ 3. I frequently think about what kind of person I am.
____ 4. When I am reading an interesting story or novel or when I am watching a good movie, I imagine how I would feel if the events in the story were happening to me.
____ 5. I seldom think about myself.
____ 6. I generally pay attention to my inner feelings.
____ 7. I value my personal self-evaluation, that is, the private opinion I have of myself.
____ 8. I sometimes step back (in my mind) in order to examine myself from a distance.
____ 9. I daydream and fantasize, with some regularity, about things that might happen to me.
____ 10. I am inclined to be introspective, that is, to analyze myself.
____ 1. When I enter a room I often become self-conscious and feel that the eyes of others are upon me.
____ 2. My thoughts are often focused on episodes of my life that I wish I’d stop thinking about.
____ 3. My nervous system sometimes feels so frazzled that I just have to get off by myself.
____ 4. I am confident about my social skills.
____ 5. Defeat or disappointment usually shame or anger me, but I try not to show it.
____ 6. It does not take me long to overcome my shyness in new situations.
____ 7. I feel relaxed even in unfamiliar social situations.
____ 8. Even when I am in a group of friends, I often feel very alone and uneasy.
____ 9. My secret thoughts, feelings, and actions would horrify some of my friends.
____ 10. I feel painfully self-conscious when I am around strangers.
____ 1. I like to be off and running as soon as I wake up in the morning.
____ 2. I’ll try anything once.
____ 3. For relaxation I like to slow down and take things easy.
____ 4. I like to wear myself out with exertion.
____ 5. I often say the first thing that comes into my head.
____ 6. I generally seek new and exciting experiences and sensations.
____ 7. I like to keep busy all the time.
____ 8. I often act on the spur of the moment.
____ 9. I sometimes do “crazy” things just to be different.
____ 10. I often feel sluggish.
How'd you do?
To find out your score for each of the four kinds of introversion, RECODE the following Reverse-Worded items: (1=5) (2=4) (4=2) (5=1):
Social Introversion items: 2, 4, & 7
Thinking Introversion item: 5
Anxious Introversion items: 4, 6, & 7
Restrained Introversion items: 1, 2, 4, 5, 6, 7, 8, & 9
Next, add together all the numbers to come up with a total score.
Here's a guide of how you scored compared to others in the general population:
- Social Introversion -- below 24 low, around 30 average, above 36 high
- Thinking Introversion -- below 28 low, around 34 average, above 40 high
- Anxious Introversion -- below 23 low, around 30 average, above 37 high
- Restrained Introversion -- below 25 low, around 31 average, above 37 high
This alternative way of assessing introversion is not likely to be embraced by Big Five personality researchers . But if it offers you a more satisfying, personally meaningful way to glean insight into your unique personality, feel free to throw the Big Five framework out the window.
© 2014 Scott Barry Kaufman, All Rights Reserved.
Acknowledgement: Thanks to Connor Child at Qzzr for his help with the online quiz, and Jennifer Odessa Grimes and Jonathan Cheek for their help with this post.
 This list is adapted from Jonathan Cheek's book review, which can be found here.
 As another example, take people who conceptualize themselves as highly introverted because they are very introspective and value their rich inner mental lives, but who score high in enthusiasm and assertiveness on the Big Five test. These folks are being told by modern personality psychologists: "You are really an extrovert who is also high in intellect/imagination." For those who have spent their entire lives equating their love of thinking and fantasy with their "introversion", they respond: "huh?" In the Big Five, imagination, fantasy, and introspection are positively associated with Extraversion. But if we do away with the label of introversion in the Big Five, then that allows a person to be introverted in the thinking/introspective sense but also be an extravert in the Big Five sense (high in enthusiasm and assertiveness).
 Popular writers on introversion are also not pleased with this psychological imperialism. For instance, in Sophia Sembling's book The Introvert's Way, she has a chapter titled "Introverts are Not Failed Extroverts".
 Keep in mind, the Big Five is a descriptive model; it merely describes patterns of covariation between people. The labels used to describe the five personality dimensions are subjective. A lot of the arguments over what counts as introversion come down to a naming game. In my view, it's really unfortunate that Big Five researchers started to use the label "introversion" to mark the lower end of extraversion. It wasn't always this way. In fact, the original name for "extraversion" in the Big Five was "Surgency". If it were up to me, it would have stayed that way, leaving the label "introversion" free to continue roaming the personality landscape. As Jonathan Cheek told me, "if the Big Five folks would just go back to that phrase ["Surgency"], they would not be crossing swords with folk psychology/ordinary language introverts. Perhaps introversion should *not* be used as a label in the Big Five system." I agree.
 Here is the link to the research report about the new STAR scale. You might be wondering: "What about the Myers-Briggs Type Indicator (MBTI) test? Isn't that good enough to measure introversion?" Well, no it isn't. As it turns out, the MBTI extraversion-introversion scale only includes items relating to being talkative, gregarious, and sociable (vs. quiet and reserved). Since there’s not a single item on the MBTI extraversion-introversion dimension that mentions being introspective or reflective, even the MBTI doesn’t measure Jung’s original conceptualization of the term!
 Big Five researchers could make the case that each of these four meanings of introversion can easily be mapped onto the Big Five framework. For instance, they could argue that:
-Social introversion is really just "low enthusiasm" (part of the extraversion domain)
-Thinking introversion is not part of the extraversion-introversion domain at all, but really is "high intellect/imagination"
-Anxious introversion is really just a blend of "high neuroticism" and "low assertiveness" (part of the extraversion domain)
-Restrained introversion" is a blend of a number of lower-order extraversion-related traits, including "low sensation seeking", "low excitement seeking", and "low activity".