I was inspired by @the-nobody-tournament's contestant 43 to write a post about selective mutism (SM). This post will include information that I have gathered through research as well as my own experiences. I will include links to some resources at the end.
SM is an anxiety disorder categorized by an inability to speak under certain (or select) circumstances, usually social settings and often when they are expected to speak. The failure to speak can appear as complete silence or whispering, and the person might use non-verbal or non-vocal methods of communication, such as signing, writing, or gesturing. Some additional symptoms of SM include fleeing from stressful social situations, crying, and physically freezing up.
SM is largely considered to be a childhood disorder, usually developing in early childhood and fading out as the child gets older, though it can develop in older persons or persist into adulthood (my own developed in my late teens). As such, most resources focus on SM in children and diagnostic guides refer to the patient as a child and focus on SM related to school.
There is no singular known cause for SM and it is considered to be multifactorial. Some theories include that it is caused by a heightened freeze response linked to social anxiety or by dissociation linked to traumatic stress.
From an exterior perspective it can appear that the person experiencing SM is choosing to be silent. In fact, it was once called elective mutism because psychologists believed that the children were choosing to be silent as a way to punish their parents. Of course, this is completely wrong - in fact, the person experiencing SM often wants to speak but is literally unable to.
When dealing with someone with SM, you should avoid putting them in highly stressful situations and be aware that their failure to speak is involuntary. The last thing you want to do is get mad at them for having SM because that can exacerbate their symptoms and lead to a total shutdown of communication.
Here are some resources on selective mutism:
American Speech-Language-Hearing Association
What is Selective Mutism by the Selective Mutism Center
Complete Guide to Selective Mutism by the Child Mind Institute (you can listen to this article)
NHS
This is not an exhaustive explanation of selective mutism and I highly suggest that you do your own research into it.
