I have heard of it and I think I had it as a child. I had no trouble speaking at home with my family, but at school I did not speak to anyone--not even other kids. When asked to read out loud in class I could not get my voice above a whisper. Most of my teachers were very understanding and since I often made the highest scores on classroom tests, they overlooked this quirk. Unlike my son, I colored in the lines really well and had nice handwriting, so school wasn't usually a problem for me. I learned to daydream to deal with boredom. I was happier daydreaming than talking to the other girls in my classes. I felt different, but I was okay with that.
But there was one middle school teacher that kept me after class and yelled at me because he couldn't hear my answers to his questions and another teacher made me feel so uncomfortable that I used to throw up before school each morning. This certainly wasn't helpful and in fact, these teachers kind of reminded me of bullies. I didn't learn well in those classes because of the anxiety. All those teachers taught me was that I was not okay the way I was.
I did manage to speak to other people as I got older when I needed to, but I would go over and over in my mind what I said thinking I sounded stupid, so I just didn't talk if I didn't have to. I was able to get good jobs that required good social skills. I married a talkative extrovert and we have a talkative extrovert child and I am a very good listener so we get along just fine.
As a child I was very close to my family, especially my mother. She was very smart and shy but not shy to the extreme that I was. It helped to have someone that understood. She never pushed me to change, so with her support I outgrew some of the social anxiety.
I have seen a few other kids, also very bright, who appear to have selective mutism--both very bright girls that I think are probably gifted.
