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.
Suppose we were to graph average SAT scores by the number of bathrooms a student has in his or her family home. That curve would also likely slope upward. (After all, people with more money buy larger homes with more bathrooms.) But it would be a mistake to conclude that installing an extra toilet raises yours kids' SAT scores.
It would be interesting to see the above graph reproduced for adopted children only. I bet that the curve would be a lot flatter."
If You Are So Rich, Why Aren't You Smart?
...The individual heritability of IQ is about 0.5: take an individual with a IQ 6 points above average and their children will be expected to have an IQ 3 points above average....Low income hinders college attendanceThe rule of thumb, I think, is that half of the income-test score correlation is due to the correlation of your test scores with your parents' IQ; and half of the income-test score correlation is coing purely from the advantages provided by that component of wealth uncorrelated with your parents' (genetic and environmental!) IQ.
...The masters at explaining this, of course, are (Googles) Samuel Bowles and Herbert Gintis, "The Inheritance of Inequality" http://www.umass.edu/preferen/gintis/intergen.pdf...
UPDATE: Conor Clarke reminds me of Christiane Capron and Michael Duyme (1989), "Assessment of Effects of Socio-Economic Status on IQ: A Full Cross-Fostering Study," Nature http://www.nature.com/nature/journal/v340/n6234/pdf/340552a0.pdf: "changes in IQ resulting from changes in postnatal environment are of similar magnitude and exhibit the same general trend independently of the SES of the adopted children's biological paretnts."
The worst scoring students from high SES families complete college as frequently as the best students from low SES families. Only 29% of high-achieving kids belonging to the lowest SES quartile obtained a bachelor's degree, compared to 74% of high-achieving kids in the top SES quartile. This success rate for high-aptitude poor students (29%) is less than the success rate for students with the lowest aptitude from the top SES families.
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