A study by the Brookings Institution states that "roughly one-quarter of the increase in student debt since 1989 can be directly attributed to Americans obtaining more education, especially graduate degrees." Their research found that since 1989, the average amount borrowed to finance a graduate degree, including a master's or doctorate diploma, increased from approximately $10,000 to over $40,000.

Workers with doctorate degrees earn some of the highest wages in the U.S . According to the Bureau of Labor Statistics (BLS), the average median weekly earnings for a person with a doctoral degree is $1,825, or about $94,900 per year. That's compared to average median weekly earnings of $1,198, or about $62,296 a year, for bachelor's degree holders and $730 a week, about $37,960 per year, for those with a high school diploma.

Between 2000 and 2017 , the percentages of Americans with a high school diploma, an associate's degree, bachelor's degree, or master's degree all increased. According to the National Center for Education Statistics (NCES), American colleges and universities conferred 1.8 million bachelor's degrees, 775,000 masters degrees and 181,000 doctorates degrees during the 2017-2018 school year.

The NCES estimates that Ph.D. graduates outside the field of education owe an average $98,800 in student loans, in part because many Americans with Ph.D.s also hold debt from earning previous degrees. The organization analyzed student loan balances of graduates from the 1999-2000 academic year to the 2015-2016 academic year and found that average loan balances for those who earned their Ph.D. increased by 104%, from $48,400 to $98,800, and student loan balances for those who earned medical doctorates increased by 97% from $124,700 to $246,000.

Average debt among students who earned other non‑Ph.D. doctorates increased by 105%, from $64,500 to $132,200. One reason for these increased debt levels among graduate students is the dramatic increase in the cost of higher education over the past several decades, for graduate as well as undergraduate students.

Though debt levels may be high, the demand for highly skilled workers is not likely to change, which means these highly-educated borrowers should continue to have an advantage in the labor market. The Georgetown Center on Education and the Workforce predicts that by 2020, 65% of all jobs in the American economy will require education beyond high school.

Despite the challenges of repaying student debt, David C. Bloomfield, professor of education leadership, law and policy at Brooklyn College and The City University of New York Graduate Center, tells CNBC Make It that earning a degree is still one of the wisest investments a student can make.

"Completed degree programs from reputable institutions," he says, "are still the most reliable investment in future income and well-being."

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