Maybe +/- 10 points is typical, but we're talking about children who are, by nature, exceptional, no?
You couldn't prove that rule by my family. I'm the third of four boys, and I got the "smart one" shingle hung around my neck pretty early on. Both of my older brothers had the opportunity to capture that one first, but nobody noticed anything unusual in them, nor in the one who came after me.
The younger one has some personality red-flags for being an unidentified giftie, though... and since at least two generations before us went completely under the radar (mom's story is a classic of underachieving rebellion), it's worth looking at.
