A new paper shows just how common it was for issuers of mortgage-backed securities to misrepresent the quality of the loans backing those bonds at the height of the housing boom. 

The paper—authored by Tomasz Piskorski, of Columbia Business School, Amit Seru, of the University of Chicago, and James Witkin, also of Columbia Business School—examined the extent to which loans that were pooled into mortgage-backed securities by private issuers with characteristics that were different from what investors were told they were getting. Such private-label securities, which weren’t guaranteed by federally supported companies Fannie Mae and Freddie Mac, had some of the worst performing loans issued during the bust.

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