High school seniors are eagerly watching their mail this month, hoping for the "fat envelope" indicating acceptance by the college of their choice. Unfortunately, more American students are receiving the "thin envelope" because college acceptance rates are continuing their decade-long decline. The most selective public colleges, such as UCLA and Michigan, now take one out of six applicants, compared to one out of three a decade ago. Top private colleges, such as Stanford and Harvard, admit one out of 20 applicants, compared to one out of 10 a decade ago. What explains the steady decline in college admission rates for...