I took a quick glance at the study. "Well off" is based on parental education, not parental income. You'd think they would be tightly correlated, but I'm not so certain..
This quote from the article, along with the burnt out shell of a VW bus sitting in a field in Mississippi, was awkward:
Lacking good information, it has been easy even for sophisticated Americans to be seduced by apologists who would have the public believe the problems are simply those of poor kids in central city schools. Our results point in quite the opposite direction. The underachievement in some southern states was a reflection of deep-rooted historical divides and disadvantages, Prof Peterson said, such as slavery and segregation.
("Seduced by apologists"?)
The outlier is Texas. Oddly, despite being part of the Confederacy, children in Texas with poorly educated parents perform inexplicably well. Of course, according to this Harvard University School of Government study, Massachusetts children are the most proficient in mathematics in the United States, second only to Germany and Switzerland...