Poverty

An achievement gap separating black from white students has long been documented — a social divide extremely vexing to policy makers and the target of one blast of school reform after another.
Only 12 percent of black fourth-grade boys are proficient in reading, compared with 38 percent of white boys, and only 12 percent of black eighth-grade boys are proficient in math, compared with 44 percent of white boys.
Poverty alone does not seem to explain the differences: poor white boys do just as well as African-American boys who do not live in poverty, measured by whether they qualify for subsidized school lunches.

http://www.nytimes.com/2010/11/09/education/09gap.html?src=me&ref=general&_r=0

Adoption by children into higher classes increases IQ.

We present a full cross-fostering study dealing with IQ, and find that children adopted by high-SES parents score higher than children adopted by low-SES parents; children born to high-SES parents score higher than children born to low-SES parents; and that there is no evidence for an interaction between these two factors on children's IQ.

http://www.nature.com/nature/journal/v340/n6234/abs/340552a0.html