Financial aid and scholarships have largely spared students from low and some middle-income families from shouldering the cost of recent tuition increases at the University of California and California State University, an independent think tank reported Wednesday.

The Public Policy Institute of California said in a new report that even though in-state tuition increased an average of 64 percent at UC campuses during the height of the recession, most families with annual incomes under $110,000 did not end up paying more thanks to a greater availability of federal and state grants and aid supplied by the colleges themselves.

The same held true for families making up to $75,000 at Cal State schools, where the average tuition rose 71 percent between 2008 and 2011.

"Essentially the pattern has been is that it's only really the highest-income students who have paid the full increase intuition," PPIC fellow Hans Johnson said.

Tuition levels have held steady at both systems for the last three years, but the institute is highlighting its findings at a time when UC leaders are thinking about raising tuition again next year and Cal State officials are asking the state for a budget increase to offset the need for another increase.

Johnson said that while students with family incomes under $30,000 are likely to remain shielded from future tuition hikes, the effect on families making more than that will depend on whether financial aid keeps pace. In the past, the universities have reserved some of the money they got from increased tuition for grants and scholarships.

He also noted that since tuition is just one of the costs of higher education, even families with low incomes can end up paying more when books, room and board are factored in. At the 23 CSU campuses, for example, the average cost of attendance for low-income students went up from $5,000 to $6,500 between 2008 and 2011 because they had to use more of their aid to cover tuition.

"Fifteen-hundred dollars might not sound like a lot, but it is to a group that is already clearly financially stressed," Johnson said.