NEW HAVEN, Conn. — Yale University said today it is joining an expanding group of elite colleges under pressure from parents, lawmakers, and alumni to make top schools more affordable to middle-class and upper middle-class families.

The Ivy League school announced what it called the largest increase in financial aid spending in its history, boosting contributions from its endowment by more than $24 million to more than $80 million annually.

The move will reduce the average cost by more than half for families with financial need, Yale officials said.

About 43% of Yale’s 5,300 undergraduates qualify for financial aid, a spokesman, Tom Conroy, said. Yale’s tuition is about $45,000 a year.

“Yale should be a college of choice for the very best and brightest students from across America and around the world, regardless of financial circumstances,” President Richard Levin said in a statement. “We want all of our students to make the most of Yale — academically and beyond — without worrying about excessive work hours or debt. Our new financial aid package makes this aspiration a reality.”

The changes go into effect in the fall.

In making the announcement Yale joins a group of elite schools — including rival Harvard — in boosting financial aid.

Harvard announced last month that it will provide $20 million more in financial aid. Harvard’s tuition, room, and board are $45,600 a year.

Duke earlier announced a plan to spend an extra $13 million on aid by capping loans and eliminating any required parental contribution from families earning under $60,000.

The University of Pennsylvania, Pomona, Swarthmore, and Haverford all announced they would eliminate loans, more than doubling the small number of schools promising all students a debt-free graduation.

Yale, which has the second biggest endowment in the nation, also announced that it would keep the increase in its tuition, room, and board charges in 2008-2009 to the expected level of consumer price inflation of 2.2%.

Yale said it is increasing the number of families who qualify for aid, eliminating the need for students to take loans, enhancing its grants to families with more than one child attending college, exempting the first $200,000 of family assets from the assessment of need, and increasing expense allowances for foreign students.

Families with incomes less than $120,000 will see their contributions cut by more than 50%, while most families with incomes between $120,000 and $200,000 will see cost reductions of 33% or more, Yale officials said. Families earning less than $60,000 will not make any contribution, while those earning between $60,000 and $120,000 will typically contribute between 1 and 10% of family income and families earning more than $120,000 will contribute an average of 10% of income.

For example, Yale said, a family with income of $90,000 would contribute $2,950 a year under the new policy, down from $12,550 under the old policy. A family making $180,000 would contribute about $23,000, down from $38,000.

Student contributions would also decline to $2,500 under the new policy from $4,400 under the old policy.

Yale officials announced last week that the school will spend more money from its $22.5 billion endowment in the 2008-2009 academic year on financial aid and scientific research. The university intends to increase the annual endowment payout for such programs by 37% to $1.15 billion, Levin said.

Yale’s endowment is topped only by Harvard’s, which stands at about $34.9 billion.