I've read literature on selective mutism written by Japanese researchers. From what I read, some recent Japanese researchers explore the relationship between selective mutism and developmental disorder.
Common diagnostic criteria, such as DSM-IV-TR and ICD-10, distinguish selective mutism from pervasive developmental disorder. If symptoms of selective mutism occurs during the course of pervasive developmental disorder, that is diagnosed as pervasive developmental disorder, not selective mutism based on anxiety. So, it is important to distinguish both disorders.
Indeed, there seem to be many mute children with pervasive developmental disorders. According to a famous Norwegian research, 68.5 per cent of selectively mute children met the diagnostic criteria for developmental disorder/delay compared with 13.0% in the control group (Kristensen, 2000).
And Shintaro Sagawa, a member of Hinode City Board of Education based on Tokyo, writes that he had seen many mute children, but they are not necessarily silent because of psychological problems. He writes some of them have many difficulties, including developmental disorder (Sagawa, 2006).
So, if you find a selectively child, it may be better to consider the possibility of pervasive developmental disorder. But Yasuhiro Watanabe and Sakakida Rie, a pediatrician and child psychiatrist, points out that it is difficult to distinguish both disorders (Watanabe and Sakaida, 2009). In case you do not know whether the child can be diagnosed as selective mutism or pervasive developmental disorder, it may be better to keep both possibilities in mind.
[Reference]
Kristensen, H. (2000). Selective mutism and comorbidity with developmental disorder/delay, anxiety disorder, and elimination disorder. Journal of the American Academy of Child & Adolescent Psychiatry. 39(2), 249-256.
Sagawa, S. (2006). Bamen kanmokuji heno approach. L'Esprit d'aujourd'hui. 471, 55-61.
Watanabe, U. and Sakakida, R. (2009). Four cases of selective mutism examined from in light of the austic spectrum disorders. Japanese journal of child and adolescent psychiatry. 50(5), 491-503.
