Selective mutism (SM) is a rare and severe social communication anxiety disorder. This is not synonymous to actual "social anxiety disorder," but at least 90% of children with SM also have a dual diagnosis of social anxiety disorder and SM. It's important to note that many people with a diagnosis of SAD do not have SM: About 7% of the general population has SAD, but only .03-1% has SM.
People with SM will usually speak at home, but they fail to initiate speech or respond verbally in envrionments where speaking is expected, such as school, even though they can speak and usually want to be able to speak and engage with others. Some with SM will not even speak in their own homes to "comfortable people" (such as parents or siblings) if there are guests over in the home. Children with SM may also struggle with speaking to parents and siblings in social settings, such as at a park or at a restaurant.
When these children feel expected to speak or pressured to speak, they become terrified. Their level of anxiety significantly increases, but by remaining silent, their anxiety level slightly decreases, obtaining some relief for themselves. For these children, remaining silent is actually an ineffective coping mechanism to maintain a sense of safety within themselves: Silence is the defense mechanism that people with SM use because they feel their best chance to maintain themselves at a baseline level of homeostasis comes with no action at all: Their mouths freeze and are "paralyzed;" they are silent.
Many of these children with SM also fear social embarrassment and have a fear of negative judgment. SM can manifest through not only paralyzed voices but also averted eye contact, behavioral inhibition, avoidance behaviors, cessation of play, and frozen nonverbals.
About .03-1.0% of the population has SM. The onset is usually prior to age 5, although many times these children are not identified until first grade or even older. Chapter 5 will be key to read as to the adverse effects on a child if not treated early and not treated properly.
Selective Mutism is
...a rare and severe social communication anxiety disorder
This disorder is not defiance, oppositional behavior, manipulative behavior, or controlling behavior, but it is anxiety. In my book, I give a thorough diagnostic rule-out procedure of other conditions that can present with similar symptoms, such as but not limited to: normative shyness, autism spectrum disorder, oppositional defiant disorder, separation anxiety disorder, social anxiety disorder, intellectual developmental disorder, and learning disability. Chapter 4 of Suffering in Silence will be important to read to understand the key differences between these different issues.