What the dimension measures
Extraversion, in the Big Five sense, describes a need for stimulation and an orientation toward the social environment. A high score goes with seeking activity, novelty and contact.
A low score does not mean fear of others. It means stimulation is reached sooner — and that beyond it, it costs rather than feeds.
What it is not
It is not shyness, which is social anxiety: one can be extraverted and shy, introverted and entirely at ease. Nor is it sociability: many introverts love company deeply, in small numbers and for bounded periods.
The distribution
Here is the point that type-based tests conceal: the real distribution is continuous and unimodal. Most people sit in the middle — sometimes called ambiversion, which is not a third type but simply the most populated zone.
Cutting at the centre separates two almost identical people into opposite categories.
The popular hypothesis of differing cortical arousal remains debated. What is solid is the measurement of the trait and its stability — not yet its neurological explanation.
What it changes
Recovery, above all. After a dense day, some need silence to become available again, others a dinner. Neither is an effort to be made: they are two ways of restoring.
It also guides travel, interiors and forms of work. RÉSONANCE records need for withdrawal and search for silence without asking, and our piece on personality tests explains why questionnaires on these subjects are fragile.