Lax loan standards have left many college graduates struggling to repay private student loans, according to a new study.

A report by the Consumer Financial Protection Bureau and the Department of Education found that the  private student loan market grew from less than $5 billion in 2001 to over $20 billion in 2008. In 2011, the figure declined to less than $5 billion as banks began to tighten credit standards and the number of undergraduates with co-signers hit a high of 90 percent.