One caveat concerning the associations in these studies relates to the restriction of range in our sample. Certainly, it is true that
individuals with average and above average cognitive ability are overrepresented in samples composed entirely of university students. [...] [O]ur participants attended a moderately selective state university. The SAT total means of our samples are roughly .60 of a standard deviation above the national mean of 1021 (College Board, 2006). The standard deviation of the distribution of scores in our sample is roughly .55–.70 of the standard deviation in the nationally representative sample.
These numbers mean that they’re dealing with the upper half of the distribution in cognitive ability among SAT takers, the mean being somewhere around the boundary between the third and the highest quartile. I strongly suspect that the correlations would be much higher if they dealt with a sample from both sides of the general population bell curve.
Note however the caveat in Note 1, i.e. the restriction of range problem. The same issue is discussed in the paper that is the source for most of these data:
These numbers mean that they’re dealing with the upper half of the distribution in cognitive ability among SAT takers, the mean being somewhere around the boundary between the third and the highest quartile. I strongly suspect that the correlations would be much higher if they dealt with a sample from both sides of the general population bell curve.