And finally, international trade and improving technology have reduced the need for low-skilled domestic labor, making college education more important than ever.

In her testimony, Hoxby said these three factors have stimulated growth and specialization in the college sector. They have also "caused educational intensity to rise in colleges that have specialized in this high intensity education." In other words, difficult colleges have become even more demanding.

In fact, recent years have not seen an overall growth in costs but rather a widening gap between expensive top colleges and cheaper, less rigorous ones, she said.

"The college sector has become much more diverse, in order to accommodate the needs of both poorly prepared high school graduates who want education akin to high school education and highly prepared students who want high intensity college. Because of this increased diversity, tuition is more diverse," Hoxby told the committee.

However, Hoxby concluded that a college education is not less accessible than it was three decades ago--in fact, she said, the opposite is true.

"Moreover, there is no evidence that students are being forced to enroll in inexpensive colleges that are inappropriate for their level of preparedness," Hoxby said.

Those who have been affected the most are not actually the lowest-income students.