For American blacks, the variance in IQ and IQ-like tests is significantly smaller than it is in whites – 65-80% as big. You already knew that the mean was ~ 1 std lower. Smaller for IQ, SAT, MCAT, bar exams, CBEST, and ACT.
I don’t know why, and I doubt if anyone does. I had a notion yesterday, but I’m not sure it works.
While I’m at it, another obscure fact: the 1-std difference exists in congenitally deaf kids. Both groups have lower verbal scores than kids with normal hearing, but differ by about 1 std.