By now, most of us are familiar with the typical introvert traits.  Introverts are likely to spend more time thinking than talking.  We avoid crowds and seek quiet.  At times we zone out when we are tired or in a large group.  These are all common introvert behaviors.  The question is, what causes introverts to behave in a certain way?  More specifically, how does introvert biology differ from extrovert biology?

To answer that, we should first discuss whether or not introversion is a product of nature or nurture.

Nature vs. nurture

A study by American psychologist, Jerome Kagan, linked introversion to high reactivity in infancy.  Four-month old babies were subjected to various forms of stimuli, including new sounds, faces and objects.  Babies who reacted dramatically to the new stimuli (crying, thrashing limbs, etc.) were defined as ‘high reactive’.

High reactive babies were found to have over-active amygdalas.  The amygdala is the emotional center where the brain triggers the adrenalin response to danger.  Put simply, the high reactive infants were easily over-stimulated.  They later became quiet, careful teenagers – introverts.

Kagan’s study shows that introversion is present from infancy.  It also suggests that the nervous system shapes personality.  Other studies of identical twins provide further evidence that introversion has a biological and genetic basis.

The introvert brain

Now that we’ve tackled the nature vs. nurture question, lets take a closer look at the biology of introversion.  In her book, The Introvert Advantage: Making the Most of Your Inner Strengths, Marti Olsen Laney talks a lot about an introvert’s nervous system.

Olsen explains that introvert traits, such as talking more slowly and showing less facial expression, can be linked to biology.  She highlights research that shows that introverts have more blood flow to the brain than extroverts.  The blood also flowed along different pathways for introverts and extroverts.

“The introverts’ blood flowed to the parts of the brain involved with internal experiences like remembering, solving problems, and planning. This pathway is long and complex. The introverts were attending to their internal thoughts and feelings.” – Marti Olsen Laney, The Introvert Advantage

The same study showed that an extrovert’s main blood pathway to the brain is short and less complex.  Extroverts paid more attention to what was happening around them in the lab.

This doesn’t mean that either personality type is more intelligent.  It does, however, suggest that introverts and extroverts process and retrieve information differently.

Introverts and dopamine

Another key difference between an introvert and extrovert’s brain has to do with dopamine.  Dopamine is a chemical that influences responses to reward and novelty.  Introverts need less dopamine to feel good.  Too much outside stimulation can cause an introvert’s brain to be flooded with an excess of the chemical.

Extroverts are less sensitive to dopamine.  They need more of the feel-good chemical to get the buzz associated with happiness.  This causes them to seek more external stimuli and rewards.

Other physiological manifestation of introversion:

  • Lower blood pressure
  • Greater sensitivity to caffeine
  • Tendency to salivate more easily
  • Sweat more in reaction to pain, smell and taste

All of this tells us that there is a biological basis for introvert behavior.  When an introvert ‘zones out’, he is actually protecting himself from overstimulation.   Being alone is a way of regulating dopamine levels.  An introvert’s internal focus can be linked to blood pathways in the brain.

What I hope people take away from this information is that introversion is a core personality trait that can’t be fixed or outgrown. Rather than trying to change us, society should recognize our innate strengths and gifts.