College Selectivity and Student Mobility

Via TC @ MR: A recent study by Caroline Hoxby

shows that although the top ten percent of colleges are substantially more selective now than they were 5 decades ago, most colleges are not more selective. Moreover, at least 50 percent of colleges are substantially less selective now than they were then. ... [S]tudents used to attend a local college regardless of their abilities and its characteristics. Now, their choices are driven far less by distance and far more by a college's resources and student body. It is the consequent re-sorting of students among colleges that has, at once, caused selectivity to rise in a small number of colleges while simultaneously causing it to fall in other colleges.

I'm a bit puzzled. Though the results almost certainly document an overall decline in student quality for a large number of more remote colleges and universities, I'm not sure this proves that there has been a decline in "selectivity" on the part of those universities. Schools may not be lowering their standards; they may just be getting a more average applicant pool with fewer top-talent students. I would invoke the principle that people will find your results more believable if you interpret them conservatively.