NEW YORK (CNNMoney) -- It's getting more difficult for low-income students to climb the economic ladder as the college graduation gap between the rich and poor grows.
While more students from all backgrounds are finishing college, the difference in graduation rates between the top and bottom income groups has widened by nearly 50% over two decades.

Some 54% of students from wealthy families obtained bachelor's degrees, said Martha Bailey, an assistant economics professor at the University of Michigan. But only 9% of low-income students got college diplomas.And since education is a key driver of upward mobility, this gulf means that it's even harder for the poor to prosper.
Bailey recently co-authored a paper looking at students who graduated in the late 1990s and early 2000s and compared them to those in college two decades before. She found the wealthy made great gains in graduation rates, while the poor only inched up over that time period.
In the earlier group, 36% of the upper-income children graduated college and 5% of the poor did.
Part of the reason is because more students from households earning at least $87,000 annually are going onto higher education. But children from families making less than $26,000 have not made the same advances, said Bailey.
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And while two-thirds of freshmen from wealthier households finish, only one-third of their poorer classmates do.
Other researchers, whose work has found similar discrepancies, have looked into why children from low-income backgrounds don't make it through college.
One reason is the poor often go to lower-tier schools, said Tim Smeeding, the director for the Institute for Research on Poverty at the University of Wisconsin-Madison. These institutions often have bigger classes and offer less individual attention and guidance.
Also, their parents don't have the financial means to aid their children.
"We've got a problem in that we get low-income kids to...