Neurodiversity Part 2:
Outline of Research on Personality

You're going to have to bear with me on this part. Because the research definition of the following terms are not the same as what we know them to be, especially from personality tests. It isn't about sociability. It's about things like cortical arousal in the brain and dopamine differences. Let me explain further.

In the 1920’s, Carl Jung famously coined the terms introversion and extroversion: the idea that some people were able to recharge more naturally with solitude while others needed social interaction to charge up. This provided the basis for the first part of the MBTI, developed from the 1940’s through the 60’s, as well as other commercialized personality tests. But it wasn’t until the availability of brain scans and studies that Jung’s observational theory could be investigated more from a physiological standpoint.

In the meantime, introversion and extroversion became synonymous with an overgeneralized notion that introverts weren’t as social as extroverts. But sociability is a much more complex and relative dimension of human behavior since it can depend on factors such as the environment a person is in, fellow participants, current mood, and past experiences, to name a few. 

However, the research into introversion and extroversion shows similarities with the public definitions of neurodiversity and neurotypical mindsets. So my goal with this article is to help define a biological model based on the following research into introversion and extroversion. This can help us understand how physiological differences can affect executive functioning and help inform future research in diagnoses and mental health variances. 

(The fun part will be explaining how those with ADHD still fit the introvert criteria. And how the stimulant medication helps to explain why, albeit in a roundabout way.)

We’re going to look at four areas in particular:

  • Blood flow: Introverts tend to have more blood flow to the frontal lobes and in the anterior thalamus (the areas used for planning, organizing information, spatial navigation, attention to task-relevant stimuli, and episodic memory), while extroverts tend to have more blood flow to the anterior cingulate gyrus, the temporal lobes, and the posterior thalamus (the areas of attention allocation, emotion association, and language comprehension.)
  • Cortical arousal: Introverts tend to have higher levels of cortical arousal while extroverts have lower levels. This helps introverts process stimuli faster, but can lead to overstimulation in hectic environments.
  • Anticipation: Extroverts showed sensitivity to anticipating rewards whereas introverts had a smaller (or no) reaction in the midst of abstract cues. Extroverts were then more likely to get a dopamine boost from anticipating a reward that had not been directly discussed. Introverts needed more context or for the reward to be spelled out.
  • Behavioral inhibition & activation systems: While introverts and extroverts can access either system, introverts tend to be more sensitive to the BIS while extroverts tend to be drawn to the BAS.


(The fun part will be explaining below how those with ADHD still fit the introvert criteria. And how the stimulant medication helps to explain why, albeit in a roundabout way.)

Also, please keep in mind that I do not mean to reduce the intricacies of these traits or the neurotransmitters / hormones I'm about to discuss down to a simple idea. But I want to use it as a starting point in order to connect what we know about neurodivergence and the research behind intro & extroversion — which can help future research into neurotransmitters and personality types.

Onto Part 3 >

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