The school district special ed team was afraid that my DS5 had selective mutism because he didn't want to speak to the speech evaluator (he does not meet the DSM criteria, however). He is extremely introverted. The website mentioned above is informative. Basically, in my ds's case, I think he's just not extreme enough to meet the criteria, though if he did it would be an extension of this introversion/anxiety (and no, I haven't been able to delineate the difference between extreme introversion as a personality trait and anxiety as a clinical pathology, though I haven't exactly consulted any psychologists on the issue. The school psychologist really didn't offer an opinion - she evaluated him too, but it was the school district speech therapist that was having a hissy fit about anxiety. long story - she admitted he has articulation issues, scoring in the single digits for percentile for artic, but she didn't seem to think it was a problem - she thought anxiety was the root cause of his speech issues. She never had to figure out what he was saying when she had no idea what was coming).
However, there is a link between introversion and IQ. I think I read that more than half of moderately gifted are introverted, and something like 75% of HG or PG (can't remember which) are introverted. In the general population, it's more like 30% or less.
As for personality characteristics, the best list I have seen is at
http://www.gifteddevelopment.com/What_is_Gifted/characgt.htm . We had our DD7 tested at this particular place about a year ago, and found them to be excellent at testing introverted kids, so when the time comes for us to test DS5, we will go there for sure. As it was, the tester said that DD was one of the most introverted kids she had ever tested. A year later, now at the end of first grade, she is really coming out of her shell, so to speak! So I guess there's hope for my DS5...
