If you think that the difference between introverts and extroverts depends only on external factors, you are wrong. Here is an article that will broaden your horizons and make you look at yourself again.
If it is necessary to start examining the differences between introversion and extroversion from infancy, it can be said that Susan Cain makes interesting findings on this subject in her work Quiet: The Power of Introverts.
Although the prejudice that an extroverted baby will be noisier sounds rational when considered with plain logic, the truth is not like this, introverts are more sensitive to external stimuli. This sensitivity makes introverted babies more reactive to change.
Physiologically, one of the key differences is dopamine
It has been a long-defended thesis in the scientific community that sensitivity to dopamine varies between introverts and extroverts. (I mentioned this before in the big five:
For example, in one study, D2 receptors were pharmacologically blocked in extrovert and introvert individuals, and introverts were found to be much more sensitive to pharmacologically induced changes in D2 receptor activity.
Therefore, while extroverts who are more sensitive to dopamine feel the need for constant stimulation in order to increase their dopamine levels, constant stimulation is tiring for introverts who are sensitive to dopamine.
On the other hand, there are theories that argue that the acetylcholine pathway, which regulates cerebral blood flow, is more active in introverts. For example, this article you can read on LinkedIn may be enlightening on this subject.
As the article suggests, both of these substances feel good, but their pathways and “ways” of feeling good are different.
Dopamine increases self-confidence, it is the reward you get when you achieve something in the outside world. Acetylcholine, on the other hand, is linked to the feeling of contentment that comes when you focus comfortably on work and spend time with intellectual activities.
From this perspective, it is not surprising that the dopamine pathway, which is more active in extroverts, is shorter because introverts need to think for a long time before even expressing an opinion.
However, “how can sensitivity – insensitivity to external stimuli be tested, other than complex methods such as blocking receptors?” If you ask and the answer you get is something like Hans Eysenck’s saliva test, it would be beneficial for you to approach this answer with caution.
The idea that the difference between introverts, who need less external stimulation, and extroverts, who need more external stimulation, can be measured by saliva production may seem logical at first, but some studies indicate that although it is logical in theory, this is not the case in practice. (an example research)
Ultimately, in order to understand the physiology of the introvert-extrovert distinction, it is rational to follow neuroscientists rather than the theories of psychologists and to examine studies conducted with methods such as fMRI and pet scan in addition to methods such as pharmacological receptor blockade.