As I have always heard it, siblings are usually within 10 IQ points of one another, barring some brain injury. That's the number I always hear thrown around. I have no source to cite though.
Anyone know where that "common knowledge" comes from?
I came across this on Hoagies, but I couldn't find the Silverman article anywhere:
8. If one child in the family is highly gifted, is it possible that the other children are gifted also?
Yes. Recent research indicated that in many cases siblings are within ten IQ points of each other (Silverman, 1987, November). If one child is highly gifted, it is quite possible that the other children are gifted, too. In many circumstances, it is beneficial for families to have all of the children evaluated.
Silverman, L K. (1987, November). Exploding the myth of the non-gifted sibling. Paper presented at the 34th Annual Convention of the Nationa1 Association for Gifted Children, New Orleans, LA.
http://www.hoagiesgifted.org/10_highly_gifted.htm
