Jungian Theory
According to Carl Jung, introversion refers to the direction of psychic energy.
If a person's energy energy normally flows inwards, this person is an introvert.
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.
Eysenck's Theory
Hans Eysenck described 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.
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 compared introversion to two of the four temperaments of ancient medicine
melancholic and phlegmatic.
Nature versus Nurture
The relative importance of nature versus environment in determining the level of introversion 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.
Brain Differences
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.
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.
Socionics
In socionics, extraversion (called "extroversion") and introversion are referred to as properties of information. These properties are grouped into two classes, elements and aspects. Aspects are the information perceived by the elements themselves, which are arranged in a person's order of information processing (called information metabolism) as functions of personality. The aspects and elements that reflect the apprehension of distinct entities (i.e. objects) are termed extroverted. Properties of object collections that are not considered distinct in themselves (fields) are termed introverted.
