This special form of a disorder of social functioning has it's onset during early childhood. Often the children show an excessive emotional sensibility. Frequently you can find other personality problems, like to withdraw from someone or something, anxiety in social situations as well oppositional behaviour.
A combination with other disorders (like the Predominantly Inattentive Type of ADHS=Attention Deficit Hyperactivity Disorder) occurs oftentimes ( more ) . One specific characteristic of selective mutism is the "refusal to speak" by children with a fear of seperation. In this case the mutism indicates the fear to be abandoned (for example after removal, divorce or illness of a close person).
The diagnostic and therapy should be done by an experienced child psychiatrist or psychologist. Self-help would not be appropriate for this kind of pathology. But the information and support of a self-help group could be very helpful.