Thirty years ago, there was a 31 percentage point difference between the share of prosperous and poor Americans who earned bachelor’s degrees, according to Martha J. Bailey and Susan M. Dynarski of the University of Michigan. Now the gap is 45 points.

While both groups improved their odds of finishing college, the affluent improved much more, widening their sizable lead.

Likely reasons include soaring incomes at the top and changes in family structure, which have left fewer low-income students with the support of two-parent homes. Neighborhoods have grown more segregated by class, leaving lower-income students increasingly concentrated in lower-quality schools. And even after accounting for financial aid, the costs of attending a public university have risen 60 percent in the past two decades. Many low-income students, feeling the need to help out at home, are deterred by the thought of years of lost wages and piles of debt…

“It’s becoming increasingly unlikely that a low-income student, no matter how intrinsically bright, moves up the socioeconomic ladder,” said Sean Reardon, a sociologist at Stanford. “What we’re talking about is a threat to the American dream.”