Studies have shown that behavioral markers for Introversion/Extraversion can be reliably detected as early as 4-months of age. Given this, a genetic basis for Introversion (as well as Extraversion) is likely.
Dopamine levels and the structure of dopamine receptors underlie this temperament difference between Introverts and Extraverts (On the Psychobiology of Personality). In broad strokes, Introverts have high baseline levels of cognitive stimulation and arousal even at rest and thus are constantly trying to avoid any additional visual and social stimuli. So when they’re trying to concentrate, nearby noises or people are additional stimuli that becomes distracting and pushes them over their optimal level of arousal. Extraverts, on the other hand, are at a constant arousal deficit and require extra stimuli to compensate and bring them to their optimal level. So they seek out places with lots of people, loud music, or interesting visuals.
When Introverts are pushed over their comfortable level of arousal, they have to bring in additional mental resources to filter out or handle the overload. This causes fatigue, and over time, they learn to avoid situations that lead to this. Extraverts are the opposite. They need environmental stimulation just to be at their optimal arousal level—i.e., the right balance between being bored and being excited. They go to parties and feel more energized from interacting with everyone and being in the midst of a lively social setting.
In Kagan’s study of 4-month old infants (Temperament In Human Nature), a noisy rattle is shaken in front of a baby. They measured how long it took until the infant became distressed and started crying. When the visual and audio stimulus from the rattle overwhelms the baby’s sensory systems, they become distressed and start crying. This happens much sooner for introverted babies than extraverted babies. Some babies start crying within 5 seconds. Others don’t get bothered at all by the rattle. This early behavioral marker is highly correlated with fear/anxiety responses at 2-years of age, and has been show to be highly correlated with physiological neural activity associated with Intro/Extraversion when these children are 10-12 years of age (PsycNET). In short, whether someone later becomes an Introvert or Extravert can be detected as early as 4-months of age.
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