Atif Mian is a professor of economics and public policy at Princeton. Amir Sufi is a professor of finance at the University of Chicago Booth School of Business.

The burden of student debt has exploded over the past decade, and economists worry that it is holding back the recovery by depressing home-buying and spending among young adults.

The federal government plays a major role in the student debt markets; 85 percent of all outstanding student loans are owed to the government. Policymakers are now debating reform of student loan programs, and their decisions will have an enormous impact on the manner in which students finance higher education.

We believe they should recognize that the central problem with student loans is that they force graduates to bear a disproportionate amount of risk for circumstances completely outside their control.

In October 2007, the unemployment rate among the previous spring’s college graduates was 8.5 percent. In October 2009, the unemployment rate for the comparable cadre of recent graduates jumped to almost 18 percent. This enormous rise was because of a nationwide economic shock — the Great Recession — that was beyond the control of any student.

In fact, when the Class of 2009 entered college in 2005, its members had good reason to be optimistic. The economy appeared strong, and job opportunities for graduates were excellent. Students took on debt believing that they could easily pay it back. But after the recession hit, many could not find jobs or accepted low wages.

You might think that the problems facing the Class of 2009 were temporary: Once the economy strengthens, they will be fine. You would be wrong. Research shows that students who graduate in an economic downturn see lower wages and worse jobs long into the future. The effects are large, negative and persistent.

But what happens to their debt obligations? Even when the job market plummets, student loans have the same principal balance and require the same interest payment. Debt does not adjust, even though the economic circumstances of the graduates have changed dramatically.

This makes no sense. Inflexible student-debt contracts place an unfair burden on young Americans. This flaw could be addressed by indexing federal student loans to the unemployment rate new graduates face.

For example, the government might use the average unemployment rate of recent graduates in normal, non-recessionary years — which a variety of estimates suggest is around 6 percent — as a benchmark. In any year when that average is higher, the government could automatically and permanently lower graduating students’ principal balance by, say, 5 percent for every percentage point that the average is above the benchmark. In this example, a student who owed $10,000 upon graduation in 2009 — when the average unemployment rate for recent graduates was 18 percent — would see a reduction in the principal balance of $6,000. A formula like this would ensure that students who graduate into the worst job markets would get the most debt relief.

This kind of contract is better because it shares the risk associated with economic downturns. And because it gives these graduates more buying power, it would address the government’s charge to stabilize the economy.

President Obama has proposed a debt reform that would make interest payments contingent on the income of the graduate, which is a step in the right direction. In a downturn, the average income of graduates would decline, and their payments would be lower.

However, that proposal provides perverse incentives that could defeat its purpose. It is based on the individual’s job and income, not aggregate economic conditions, so a graduate could lower his interest payment by taking a lower-paying job, or perhaps by turning a job down. A program that automatically lowered all student-debt burdens in a weak economy would better preserve incentives as well as risk-sharing.

Closer to our idea is a proposal from Sen. Elizabeth Warren (D-Mass.) that would allow students to refinance into lower rates. It also would act as a type of insurance, because rates tend to fall in recessions. We would support this proposal but, as we’ve seen with the mortgage market, refinancing doesn’t always work well, particularly for individuals with low credit scores. Some former students won’t be aware of the opportunity. Others might not find a willing lender. We worry that a large number of eligible students would not be able to use this proposal.

In our indexing proposal, interest payments would automatically come down for all students when economic conditions deteriorate. It would not rely on graduates seeking refinancing.

Further, we believe that graduates should get more than just an interest-rate reduction if economic conditions deteriorate sufficiently — principal, too, should be forgiven. For example, the government could forgive the debts of students who graduated in 2008, 2009 and 2010 — in a horrible job market.

Student debt as currently structured is flawed because it forces students to bear undeserved risk. Many students understand that this risk is large and may therefore avoid going to college altogether. This is a disastrous outcome. We need a better student loan program that provides both the opportunity to go to college and insurance in case the economy collapses.