Introversion and extroversion

The terms introvert and extrovert (spelled extravert by Carl Jung) were originally employed by Sigmund Freud and given significant amplification later by Jung. The terms refer to "attitudes" and show how a person orients and receives their energy.

In the extroverted attitude the energy flow is outward, and the preferred focus is on people and things. Extroverts tend to be energetic, enthusiastic, action-oriented, talkative, and assertive. Therefore an extroverted person is likely to enjoy time spent with people and find less reward in time spent alone.

In the introverted attitude the energy flow is inward, and the preferred focus is on thoughts and ideas. Introverts tend to be quiet, low-key, deliberate, and disengaged from the social world. Thus, one who is introverted is more likely to spend time alone or in contemplation, as these activities are rewarding. They may avoid social situations entirely, not because of shyness, but because they choose to.

While most people view being either introverted or extroverted as a question with only two answers, the reality is that this is probably more of a scale, with people falling on both ends and in between. The term ambivert was coined to denote 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. Ambiverts tend to be moderate thinkers and weigh more than one side to an issue. Most have warm but controlled personalities.

In today's society it is generally seen as more of a positive quality to lean towards being extrovert rather than introvert. Most people who consider themselves introverts usually steer clear of the word when describing themselves -- e.g., at a job interview, because they think people will see them as eccentric and different. People who see themselves as extroverts however, will gladly use this word in a description of themselves thinking most people will consider it a positive quality.

Causes
The relative importance of nature versus environment in determining the level of introversion/extroversion 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 (not shared by siblings).

Hans Eysenck proposed that extroversion was caused by variability in cortical arousal; "introverts are characterized by higher levels of activity than extroverts and so are chronically more cortically aroused than extroverts". Because extroverts are less aroused internally, they require more external stimulation than introverts. This theory may be backed up by evidence that extroversion is tied to a gene that shapes the brain's response to dopamine. Other evidence of this “stimulation” hypothesis is that introverts are less tolerant of painful electric shock and salivate more than extroverts

One study found that introverts have more blood flow in the frontal lobes of their brain and the anterior or front thalamus, which are areas dealing with internal processing such as remembering and problem solving. Extroverts have more blood flow in anterior cingulate gyrus, temporal lobes and posterior thalamus, which are involved in sensory processing such as listening and watching. It is difficult to determine the causal relationship in this case. The differences in brain activity may cause the differences in personality, or the person's tendency to introversion/extroversion may mainfest itself in brain activity, or there may be some complex interaction between the two.