There are the following similarities between introvert personality type, Social anxiety disorder and mild autism
How to distinguish these conditions are due to introvert personality, social anxiety disorder and mild autism?
There is an ongoing debate as to whether it is best to view personality disorders as being categorical or dimensional in nature. Your question infers the perspective of the categorical view. The alternative dimensional perspective suggests that the three disorders you mention are very similar, that is, these three conditions overlap to a large extent with a slight shift along an (as yet unnamed) dimension.
Hopwood, C. J., Kotov, R., Krueger, R. F., Watson, D., Widiger, T. A., Althoff, R. R., Ansell, E. B., Bach, B., Michael Bagby, R., Blais, M. A., Bornovalova, M. A., Chmielewski, M., Cicero, D. C., Conway, C., De Clercq, B., De Fruyt, F., Docherty, A. R., Eaton, N. R., Edens, J. F., … Zimmermann, J. (2018). The time has come for dimensional personality disorder diagnosis. Personality and Mental Health, 12(1), 82–86. https://doi.org/10.1002/pmh.1408
The 3 said conditions look very similar, but their meaning is different. but they indicate different parameters or different directions. Overlap can happen, such as an "introvert autistic" or an "extrovert autistic"
Introversion is an aversion or avoidance of social situations.
Social anxiety is a fear of social situations.
Autism is an extremely broad range of conditions, which may include inability to process social information.
How does they differ?
Introversion: It is sort of very old term, very broad and general one.
Introversion does not necessarily include an inability to process social information. Rather it is seen that many introverts are better in predicting how others feel at social context.
Introversion does not also necessarily mean a fear and/or shyness of social situation.
Social anxiety disorder (Social phobia) means a fear of social situation.
Social anxiety does not necessarily mean a lack of social understanding.
Social anxiety may be seen from very young age or may develop at a later age due to trauma, stress, etc.
Social anxiety disorder typically starts in childhood or adolescence. Among individuals who seek treatment as adults the median age of onset is in the early to mid-teens with most people having developed the condition before they reach their 20s. However, there is a small subgroup of people who develop the condition in later life. Some people can identify a particular time when their social anxiety disorder started and may associate it with a particular event (for example, moving to a new school or being bullied or teased).
--- Social Anxiety Disorder: Recognition, Assessment and Treatment. NICE Clinical Guidelines, No. 159. National Collaborating Centre for Mental Health (UK).Leicester (UK): British Psychological Society; 2013. NCBI Bookshelf
Autism seemingly is a genetically determined set of conditions, although it invites bullying, discrimination, hatred etc that may further intensify social anxiety.
The word autistic comes from the word "autos" that mean "self"; but this self-limiting does not necessarily comes from an aversion of people, but it generally comes from an inherent lack of social understanding. There may be deficit of understanding other people's intentions and mindsets ("lack of cognitive empathy") or may have an unusual or different "theory of mind"; may have context blindness i.e. an inability to process how the same task evoke different reactions at different social contexts.
Autistics may have a problem understanding or using nonverbal communication such as posture, gesture, proxemics, etc.
Autistic people often want friend, and often forge for friend. The social aversion or friendship aversion may be secondary; due to misunderstandings, failed communications, rejection, trauma etc.
Autism often comes with one or more co-morbid disabilities like dyslexia, dyscalculia, dysgraphia, developmental coordination disorder, pragmatic language disorder, nonverbal learning disorder, prosopagnosia etc.
Notably, Due to pressure from society, autistics may develop various masking mechanisms and tricks (such as laughing when everybody laughs, force maintain eye contact, force greetings etc) so that they appear less neurodivergent.
The take-home message is that, the psychologist/ psychiatrist who have to distinguish the 3 conditions need to be utmost cautious and aware not just the behaviour but the inherent cognitive 'reason' behind the symptoms.
Here is an article which is very helpful for finding information about What is Avoidant personality disorder & social anxiety. I personally found it very informative because i have seen that many people around me who suffered. https://sympdisease.com/what-is-avoidant-personality-disorder-social-anxiety/#Is_avoidant_personality_disorder_an_anxiety_disorder
