Selective colleges get plenty of applications from the top-scoring children of affluent parents, including many in this region. What the colleges need, their admissions officers say, are more high-achieving, low-income applicants. Places such as Georgetown and Duke don’t like being called country clubs for the rich. They want more academically talented poor kids. Why aren’t they applying? Now we know, thanks to remarkable research by economists Caroline Hoxby of Stanford and Sarah Turner of the University of Virginia.