Many Japanese involving selective mutism view that selective mutism is an "emotional disturbance."
I guess that view derives from government's policy on special education. Director-General, Elementary and Secondary Education Bureau, Ministry of Education, Culture, Sports, Science and Technology (MEXT) notified "About school children and students with disabilities" in 2002. In the notification, children with selective mutism is an object of education of emotional disturbance children.
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Children who can benefit from special speech classes
1 Autistic Children, etc. who can mostly study in normal classes and partly need special guidance.
2 Children with selective mutism, etc. mainly due to psychogenic factors who can mostly study in normal classes and partly need special guidance.
(from "About school children and students with disabilities")
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The authorities included selective mutism in emotional disturbance at least 41 years ago (1967), as far as I know. In 1967 Education Ministry conducted "An investigation on school children and students' psychosomatic disorder" and defined emotional disturbance as school refusal, neurosis, selective mutism, autism, mental illness, organic brain disorder, etc.
Index of SM in Japan
