In response to a NY Times column with a graph showing that kids from higher income families score higher on the SAT, Harvard professor Greg Mankiw writes:
This graph is a good example of omitted variable bias, a statistical issue discussed in Chapter 2 of my favorite textbook. The key omitted variable here is parents' IQ. Smart parents make more money and pass those good genes on to their offspring.
In contrast, liberals believe that rich parents are buying something with their money which causes higher SAT scores.