From the Linda Silverman article, What We Have Learned About Gifted Children
30th Anniversary 1979 - 2009
Brothers and sisters are usually within five or ten points in measured ability. Parents' IQ scores are often within 10 points of their children's; even grandparents' IQ scores may be within 10 points of their grandchildren's. We studied 148 sets of siblings and found that over 1/3 were within five points of each other, over 3/5 were within 10 points, and nearly 3/4 were within 13 points. When one child in the family is identified as gifted, the chances are great that all members of the family are gifted.
This still leaves a significant number of siblings (25%) who are more than 13 points apart. I've only had my 2e kid tested so I don't know how the siblings compare.
