Politicians on both sides of the aisle want to make the justice system more equitable. Part of that discussion includes the problem of wrongful convictions. WGBH’s Morning Edition Host Joe Mathieu talks with Northeastern University law professor and WGBH News legal analyst Daniel Medwed about the scope of wrongful convictions. The transcript below has been edited for clarity.

Joe Mathieu: You've made wrongful convictions the focal point of your academic research. What prompted your interest in this to begin with?

Daniel Medwed: Before I became a professor, I was a criminal defense lawyer in New York. First as a public defender handling appeals for indigent defendants who were dissatisfied with their convictions. And then I ran a small clinic at Brooklyn Law School, where my students and I investigated and litigated claims of innocence by New York state prisoners. What those experiences taught me is that the phenomenon of wrongful convictions — of actually innocent people being convicted either at trial or through plea bargains — is much more widespread than I ever imagined. So I naturally gravitated to that research area when I became a professor.

Mathieu: How widespread are we talking here? What kind of numbers do you have? How many prisoners have proven their innocence?

Medwed: You know, that's the $64,000 question. I guess with inflation it's much higher than that now. Here's what we know: according to the Innocence Project in New York, there have been 365 documented DNA exonerations of innocent prisoners across the country since 1989. Those are cases where post-conviction DNA testing of biological material retained from the crime scene has been tested and excluded the person who was convicted of the crime as being the perpetrator. Twenty of those people were on death row at the time. Now biological evidence is seldom available in a criminal case, so we typically see exonerations in non-DNA cases — without the magic bullet of science — and scholars have pin-pointed about 2,100 other non-DNA exonerations across the country since 1989.

Mathieu: Wow. How many of those cases involve prisoners in Massachusetts?

Medwed: Well, when you deconstruct that data set, it appears as though there are 65 well-documented exoneration cases from Massachusetts, including 10 DNA exonerations — again where post-conviction DNA testing excluded the person convicted of the crime as the culprit. Most of those DNA exonerations involved rape or murder, which are the types of cases that would leave biological evidence at the crime scene.

Mathieu: Those are just proven cases, Daniel. [Is there] any sense about how many innocent prisoners might still be behind bars?

Medwed: Well, there's a sense that these proven cases are just the tip of the innocence iceberg. And here's why. First, if you're claiming innocence behind bars, you need help to investigate and litigate your case. And there just aren't that many lawyers out there that do this kind of work. There are two organizations in Massachusetts. One [is] the New England Innocence Project. [It's] a nonprofit 501c3 [and] operates on a shoestring. For purposes of full disclosure, I'm on the board of that group. There's also an innocence program affiliated with our state public defenders: [The] Committee for Public Counsel Services. Other than that, there might be a few individual lawyers who take on cases pro bono, but there aren't [a lot] of people out there to provide help.

Second, even if you can scramble and find someone to represent you, you're facing a number of practical obstacles in proving your innocence. For one thing, a lot of these cases are old and cold. You need to find newly discovered evidence of innocence to overturn your conviction. Witnesses might be hard to find, memories are stale, evidence might be destroyed and so on. And finally, there are [a lot] of procedural obstacles, because once you've been convicted a presumption of guilt takes root, and you have to possibly circumvent certain statutes of limitations or other procedural hurdles like limitations on the number of things that you can file.

Mathieu: What does that mean for Massachusetts?

Medwed: Well, Massachusetts has the lowest incarceration rate in the country — about 126 people per 100,000 residents, which is much lower than, say, Louisiana and other states by multiples. But even if you have a one percent error rate — which is a very conservative estimate — we're talking over 80 prisoners in Massachusetts, because there are 8,200 prisoners in our state facilities. And then even more in our city and county jails.