An increasing amount of research in recent years casts doubt upon the idea that cognitive ability is spread evenly among all ethnicities.
No "White" Child Left Behind: The Academic Achievement Gap between Black and White Students
Rowley, Rochelle L.; Wright, David W.
Journal of Negro Education, v80 n2 p93-107 Spr 2011
Alzheimer’s Disease In African Americans: Risk Factors And Challenges For The Future
Lisa L. Barnes and David A. Bennett
Health Aff (Millwood). 2014 Apr; 33(4): 580–586