Let me put on my psychology researching hat to answer this…
I want to preface this response by saying that I knew little about selective mutism until I did research on it. So please take what I say with a grain of salt. Also, it’s important to mention that clinical diagnostics and research are all I can comment on, as I have not heard secondhand any personal experiences with selective mutism. It would be great if those were involved here too.
For a quick background, I went to Wikipedia. I will bold the parts relevant to your question.
According to a more recent systematic study it is believed that children who have selective mutism are not more likely than other children to have a history of early trauma or stressful life events. Another recent study by Dummit et al., in 1997 did not find any evidence of trauma in their sample of children. Recent evidence has shown that trauma doesn’t explain why most children with selective mutism develop the condition. Many children who have Selective Mutism almost always speak confidently in some situations. Children who have experienced trauma however are known to suddenly stop speaking.
The Diagnostic and Statistical Manual of Mental Disorders (DSM), first published in 1952, first included elective mutism in its third edition, published in 1980. Elective mutism was described as “a continuous refusal to speak in almost all social situations” despite normal ability to speak. While “excessive shyness” and other anxiety-related traits were listed as associated features, predisposing factors included “maternal overprotection”, mental retardation, and trauma. Elective mutism in the third edition revised (DSM III-R) is described similarly to the third edition except for specifying that the disorder is not related to social phobia.
In 1994, Sue Newman, co-founder of the Selective Mutism Foundation, requested that the fourth edition of the DSM reflect the name change from elective mutism to selective mutism and describe the disorder as a failure to speak. The relation to anxiety disorders was emphasized, particularly in the revised version (DSM IV-TR).
Maya Angelou’s breakout award winning memoir “I Know Why The Caged Bird Sings”, chronicles her mutism following repeated rapes at age eight by her mother’s boyfriend. After the trial he was murdered and Ms. Angelou fell silent for years feeling responsible.
Adoption Detective by Judith Land mentions selective mutism, extreme shyness, and other social anxiety disorders as evidence of trauma frequently associated with adoption. She explains that adoptees with selective mutism have difficulty verbalizing personal thoughts that are excessively revealing and painful or of a subconscious nature; selective mutism can be highly functional for a child by reducing anxiety and protecting the child from perceived challenges of social interaction.
So it seems like trauma can be a prerequisite to selective mutism, but most children with selective mutism have not experienced trauma.
I pulled out my DSM-IV-TR to look at the entry for Selective Mutism (313.23). It mentions psychosocial stressors being associated with selective mutism. Psychosocial stressors can be any number of things, including abuse and trauma.
Associated features of Selective Mutism may include excessive shyness, fear of social embarrassment, social isolation and withdrawal, clinging, compulsive traits, negativism, temper tantrums, or controlling or oppositional behavior, particularly at home. There may be severe impairment in social and school functioning. Teasing or scapegoating by peers is common. Mental Retardation, hospitalization, or extreme psychosocial stressors may be associated with the disorder. In addition, in clinical settings, children with Selective Mutism are almost always given an additional diagnosis of an Anxiety Disorder (especially Social Phobia).
As for autism, the DSM doesn’t mention it specifically in relation to selective mutism. However, it does say that if the mutism occurs “exclusively during the course of a Pervasive Developmental Disorder” then that diagnosis will be given and Selective Mutism will be abandoned as a diagnostic option. In this version of the DSM, Autism Disorder is considered a Pervasive Developmental Disorder.
The Wikipedia article mentions autism, specifically. I take Wikipedia with a grain of salt, always (and this section doesn’t have a citation):
Particularly in young children, SM can sometimes be confused with an autism spectrum disorder, especially if the child acts particularly withdrawn around their diagnostician, which can lead to incorrect treatment. Although autistic people may also be selectively mute, they display other behaviors—hand flapping, repetitive behaviors, social isolation even among family members (not always answering to name, for example)—that set them apart from a child with selective mutism. Some autistic people may be selectively mute due to anxiety in social situations that they do not fully understand. If mutism is entirely due to autism spectrum disorder, it cannot be diagnosed as selective mutism.
Sooooo TL;DR - I don’t know why the OP said selective mutism cannot be caused by trauma or abuse, but it’s clear that it is not caused by autism.
Thanks for the ask! -MO