Washington - French economist Thomas Piketty has joined the chorus of experts who think that student debt in the United States, if gone unchecked, could undermine economic growth in this country.

"If we really want to promote more equal opportunity and redistribute chances in access to education we should do something about student debt," Piketty said in an interview posted Sunday on Big Think. "This is really the key for higher growth in the future and also for a more equitable growth."

The meteoric rise of student debt in the United States has led economists and policymakers to question whether it could erode economic gains and exacerbate inequality.

There are roughly 43 million Americans with outstanding student debt, and many are having a hard time paying it off. The number of late payments on student loans has crept up in the last few months. And it's happening at a time when Americans are doing a good job of paying down other types of consumer credit, like mortgages and credit cards, according to the Federal Reserve Bank of New York.

Many economists think that hefty student debt is unlikely to topple the economy and lead to another recession, but it could affect consumer behavior in a way that is ultimately detrimental to sustainable economic growth.

"How we finance post-secondary education has significant effects on a variety of critical economic outcomes, including economic growth and inequality," New York Fed President William Dudley said at a recent event on student loan data. "Higher student debt and delinquencies reduce household formation and depress home ownership."

What's concerning is the amount of time people are taking to repay their student loans. Economists Meta Brown and Andrew Haughwout at the New York Fed found that only 17 percent of the debt owed by 2009 graduates had only been paid down after five years. Graduates who left school 10 years ago had barely paid down a third of the money they owed.

It's not completely clear how having large amounts of debt for an extended period affects borrowers' decisions. But the economists say Fed research has shown that student debt is contributing to millennials delaying homeownership and camping out at mom and dad's for longer than prior generations. And that is holding back consumer spending.

Beth Akers and Matthew Chingos of the Brookings Institute have argued, however, that the pace with which people repay student debt and their monthly payments are not that different compared to 20 years ago. They think that student debt, therefore, may not hold the economy back in the long run, especially as borrowers get jobs that make it easier for them to buy homes and cars. Having a college degree, after all, generally increases a person's lifetime earnings.

The trouble is that there are a growing number of people who are not graduating, but are leaving school with mounds of debt. The chances of them finding jobs to pay off what they owe are slim as more employers require college degrees for even entry-level positions.