Extrovert
The trait of extraversion-introversion is a central dimension of human personality. Extraverts (also spelled extroverts) tend to be gregarious, assertive, and interested in seeking out excitement.  Introverts, in contrast, tend to be more reserved, less outgoing, and less sociable.  They are not necessarily loners but they tend to have smaller circles of friends and are less likely to thrive on making new social contacts. Introverts are less likely to seek stimulation from others because their own thoughts and imagination are stimulating enough.
The terms introversion and extraversion were first popularized by Carl Jung. Virtually all comprehensive models of personality include these concepts.  Examples include Jung's Analytical psychology, Eysenck's three-factor model, Cattell's 16 personality factors, the Big Five personality traits, the four temperaments, the Minnesota Multiphasic Personality Inventory, the Myers Briggs Type Indicator, and Socionics.
Extraversion and introversion are generally understood as a single continuum.  Thus, to be high on one is necessarily to be low on the other.  That said, people fluctuate in their behavior all the time, and even extreme introverts and extraverts do not always act consistently.

# Varieties

## Extraversion
Extraversion is "the act, state, or habit of being predominantly concerned with and obtaining gratification from what is outside the self". Extraverts tend to enjoy human interactions and to be enthusiastic, talkative, assertive, and gregarious. They take pleasure in activities that involve large social gatherings, such as parties, community activities, public demonstrations, and business or political groups. Politics, teaching, sales, managing, and brokering are fields that favor extraversion. An extraverted person is likely to enjoy time spent with people and find less reward in time spent alone. They tend to be energized when around other people, and they are more prone to boredom when they are by themselves.

## Introversion
Introversion is "the state of or tendency toward being wholly or predominantly concerned with and interested in one's own mental life". Introverts tend to be more reserved and less assertive in social situations. They often take pleasure in solitary activities such as reading, writing, drawing, and using computers. The archetypal artist, writer, sculptor, composer, and inventor are all highly introverted. An introvert is likely to enjoy time spent alone and find less reward in time spent with large groups of people, though they tend to enjoy interactions with close friends. They prefer to concentrate on a single activity at a time and like to observe situations before they participate. Introverts are easily overwhelmed by too much stimulation from social gatherings and engagement. They are more analytical before speaking.
Introversion is not the same as shyness. Introverts choose solitary over social activities by preference, whereas shy people avoid social encounters out of fear.

## Ambiversion
Although many people view being introverted or extraverted as a question with only two possible answers, most contemporary trait theories (e.g. the Big Five) measure levels of extraversion-introversion as part of a single, continuous dimension of personality, with some scores near one end, and others near the half-way mark. Ambiversion is a term used to describe people who fall more or less directly in the middle and exhibit tendencies of both groups. An ambivert is normally comfortable with groups and enjoys social interaction, but also relishes time alone and away from the crowd

# Measurement
Extraversion-introversion is normally measured by self-report. A questionnaire might ask if the test-taker agrees or disagrees with statements such as I am the life of the party or I think before I talk.
Imagine a questionnaire consisting of ten "agree or disagree" statements. For the first five questions, agreement indicates a tendency towards extraversion, while for the last five questions, agreement indicates introversion. Five people take this questionnaire and answer as follows:
In this example, John and Maria are extraverted, Sarah and David are introverted, and Marcus is neither.
Self-report questionnaires have obvious limitations in that people may misrepresent themselves either intentionally or through lack of self-knowledge. It is also common to use peer report or observation.
Another approach is to present test-takers with various sets of adjectives (for example: thoughtful, talkative, energetic, independent) and ask which describes them most and least. Psychological measures of this trait may break it down into subfactors including warmth, affiliation, positive affect, excitement seeking, and assertiveness/dominance seeking.

# Causes

## Jungian theory
According to Carl Jung, introversion and extraversion refer to the direction of psychic energy. If a persons psychic energy usually flows outwards then he or she is an extravert, while if the energy usually flows inwards, the person is an introvert. Extraverts feel an increase of perceived energy when interacting with a large group of people, but a decrease of energy when left alone. Conversely, introverts feel an increase of energy when alone, but a decrease of energy when surrounded by a large group of people.
Most modern psychologists consider theories of psychic energy to be obsolete. First, it is difficult to operationalize mental "energy" in ways that can be scientifically measured and tested. Second, more detailed explanations of extraversion and the brain have replaced Jung's rather speculative theories. Nevertheless, the concept is still in popular usage in the general sense of "feeling energized" in particular situations. Jungs primary legacy in this area may be the popularizing of the terms introvert and extravert to refer to a particular dimension of personality.

## Eysenck's theory
Hans Eysenck described extraversion-introversion as the degree to which a person is outgoing and interactive with other people. These behavioral differences are presumed to be the result of underlying differences in brain physiology. Extraverts seek excitement and social activity in an effort to heighten their arousal level, whereas introverts tend to avoid social situations in an effort to keep such arousal to a minimum. Eysenck designated extraversion as one of three major traits in his P-E-N model of personality, which also includes psychoticism and neuroticism.
Eysenck originally suggested that extraversion was a combination of two major tendencies, impulsiveness and sociability. He later added several other more specific traits, namely liveliness, activity level, and excitability. These traits are further linked in his personality hierarchy to even more specific habitual responses, such as partying on the weekend.
Eysenck compared this trait to the four temperaments of ancient medicine, with choleric and sanguine temperaments equating to extraversion, and melancholic and phlegmatic temperaments equating to introversion.

## Biological factors
The relative importance of nature versus environment in determining the level of extraversion is controversial and the focus of many studies. Twin studies find a genetic component of 39% to 58%. In terms of the environmental component, the shared family environment appears to be far less important than individual environmental factors that are not shared between siblings.
Eysenck proposed that extraversion was caused by variability in cortical arousal. He hypothesized that introverts are characterized by higher levels of activity than extraverts and so are chronically more cortically aroused than extraverts. The fact that extraverts require more external stimulation than introverts has been interpreted as evidence for this hypothesis. Other evidence of the "stimulation" hypothesis is that introverts salivate more than extraverts in response to a drop of lemon juice.
Extraversion has been linked to higher sensitivity of the mesolimbic dopamine system to potentially rewarding stimuli. This in part explains the high levels of positive affect found in extraverts, since they will more intensely feel the excitement of a potential reward. One consequence of this is that extraverts can more easily learn the contingencies for positive reinforcement, since the reward itself is experienced as greater.
One study found that introverts have more blood flow in the frontal lobes of their brain and the anterior or frontal thalamus, which are areas dealing with internal processing, such as planning and problem solving. Extraverts have more blood flow in the anterior cingulate gyrus, temporal lobes, and posterior thalamus, which are involved in sensory and emotional experience. This study and other research indicates that introversion-extraversion is related to individual differences in brain function.

# Behavior
Extraverts and introverts have a variety of behavioral differences. According to one study, extraverts tend to wear more decorative clothing, whereas introverts prefer practical, comfortable clothes. Extraverts are likely to prefer more upbeat, conventional, and energetic music than introverts. Personality also influences how people arrange their work areas. In general, extraverts decorate their offices more, keep their doors open, keep extra chairs nearby, and are more likely to put dishes of candy on their desks. These are attempts to invite co-workers and encourage interaction. Introverts, in contrast, decorate less and tend to arrange their workspace to discourage social interaction.
Although extraverts and introverts have real personality and behavior differences, it is important to avoid pigeonholing or stereotyping by personality. Humans are complex and unique, and because extraversion varies along a continuum, they may have a mixture of both orientations. A person who acts introverted in one scenario may act extraverted in another, and people can learn to act against type in certain situations. Jung's theory states that when someone's primary function is extraverted, his secondary function is always introverted (and vice versa).

# Mental health
Acknowledging that introversion and extraversion are normal variants of behavior can help in self-acceptance and understanding of others. For example, an extravert can accept her introverted partners need for space, while an introvert can acknowledge his extraverted partners need for social interaction.
Social psychologist David Myers found a correlation between extraversion and happiness; that is, more extraverted people reported higher levels of personal happiness. The causality is not clear, however. Extraversion may lead to greater happiness, happier people may become more extraverted, or there may be some other factor such as genetics that affects both. It is also possible that the results reflect biases in the survey itself. Another factor is that introversion is generally regarded as less healthy in Western culture. Also, according to Carl Jung, introverts acknowledge more readily their psychological needs and problems, whereas extraverts tend to be oblivious of them because they focus more on the outer world. On average, extraverts also have a somewhat higher self-esteem than introverts. As in the case of happiness, this may be due to inherent differences in the brain, or differential social treatment.
Extraversion is perceived as socially desirable in Western culture, but it is not always an advantage. For example, extraverted youths are more likely to engage in delinquent behavior.Conversely, while introversion is perceived as less socially desirable, introversion is strongly associated with positive traits such as intelligence and "giftedness." For many years, researchers have found that introverts tend to be more successful in academic environments, which extraverts may find boring.
Career counselors often use personality traits, along with other factors such as skill and interest, to advise their clients. Some careers such as computer programming may be more satisfying for an introverted temperament, while other areas such as sales may be more agreeable to the extraverted type.
Although neither introversion nor extraversion is pathological, psychotherapists can take temperament into account when treating clients. Clients may respond better to different types of treatment depending on where they fall on the introversion/extraversion spectrum. Teachers can also consider temperament when dealing with their pupils, for example acknowledging that introverted children need more encouragement to speak in class while extraverted children may grow restless during long periods of quiet study.

# Geography
Researchers have found that people living in the midwestern states of North Dakota, South Dakota, Nebraska, Minnesota, Wisconsin, and Illinois score higher than the U.S. average on extraversion. Utah and the southeastern states of Florida and Georgia also score high on this personality trait. The most introverted states in the United States are Maryland, New Hampshire, Alaska, Washington, and Vermont. People who live in the northwestern states of Idaho, Montana, and Wyoming are also relatively introverted.