<p>Tom Mortenson of "College Opportunities" (<a href="http://www.postsecondary.org%5B/url%5D">www.postsecondary.org</a>) has been studying this question (though not just with top colleges) for almost 20 years, as well as providing evidence (with many outlyers of course) of the straight-line inverse distribution associating 6-year graduation rates with family incomes of $60k and under, as well as federal studies on reasons cited for interruptions in higher education.</p>
<p>Now, it should be remembered, that a $60k/year family income puts one toward the top of the middle quintile in family income in the U.S. Mortenson notes (on his blog) the association between graduation rates and selectivity, but that class-based criteria strongly govern who actually gets in. Based on his data, he calls these schools "the gated communities of higher education."</p>
<p>Carolyn Lawrence has a link to her site <a href="http://www.collegehunt.blogspot.com/%5B/url%5D">http://www.collegehunt.blogspot.com/</a> to a government website that shows not only the percentage of those who receive need-based aid, but also a distribution of family incomes of those who apply for federal aid, with breaks at $40k and $60k/yr. By adding together the percentage of Pell Grant recipients, with those in the $40-$60k range, one can quickly get a sense of those most likely not to graduate for economic/family reasons. You also quickly discover the correlation between small percentages in this range and high graduation rates, with schools like Williams, Yale, Trinity, Middlebury at the forward end of the chart.</p>