Why? There’s no definitive answer. However, wrongful conviction experts offered several explanations for the 13 outliers.

Several states have bad track records of preserving evidence:

Three of the 13 states — Delaware, North Dakota, and Vermont — don’t have evidence preservation laws, and four of them don’t require evidence to be preserved for the length of incarceration. For many prisoners seeking to overturn their convictions, the right to DNA evidence testing exists in theory but not in practice. Iowa is among 13 U.S. states where no prisoner has ever successfully used DNA testing to win exoneration. One of those four states is Iowa, where, as my colleague Abigail Blachman reported, wrongful conviction attorneys often find that police have destroyed or misplaced DNA evidence that authorities could have tested. Most states passed evidence preservation in the past 10-15 years, but that doesn’t help many inmates who had been languishing in prison, said Brandon L. Garrett, director of the Center for Science and Justice at Duke University School of Law. “This has been an important change, but there are so many people who might have been freed by a DNA test if evidence had been preserved in the 1970s, ‘80s and ‘90s,” Garret said. “We will never know how many innocent people could have been freed.” The laws also don’t ensure evidence technicians interpret DNA samples properly, promise proper preservation for the length of a person’s prison term, or protect evidence vaults from extreme weather and natural disasters. Moreover, many police departments still routinely dispose of evidence after a conviction. Some of the 13 states were late to adopt laws providing prisoners the right to post-conviction DNA testing, like Alaska and South Dakota, among the last states to pass post-conviction DNA testing laws in 2010 and 2009, respectively. Defendants convicted before those laws passed may be out of luck.

The high bar for post-conviction DNA testing could be a factor:

Some of the states have really high standards for post-conviction DNA testing, said Michelle Feldman, state campaigns director at the Innocence Project, a national nonprofit focused on exonerating wrongfully convicted people. For example: Alaska requires a person to not have pled guilty, although about 1 in 10 exonerations have involved guilty pleas, according to the National Registry of Exonerations, a project maintained by several universities and law schools.
Arkansas, Delaware and New Hampshire require that a prisoner prove their innocence or show DNA testing will implicate someone else in the crime before DNA testing is allowed, requiring petitioners to essentially solve the crime they’re accused of. Oregon, one of two states where courts can convict a person without all 12 jurors agreeing on a verdict, had similar provisions in its post-conviction DNA testing law. But earlier this year, the state legislature amended the statute to allow testing if there’s a reasonable probability DNA testing would have prevented the petitioner’s conviction.
“The whole point of getting DNA tested is to see what the results are and then show why that proves you didn’t commit the crime,” Feldman said. Vanessa Potkin, director of post-conviction litigation at the Innocence Project, named several factors that might affect an inmate’s ability to access post-conviction DNA testing: limited access to post-conviction counsel and public defender systems for indigent defendants, and Innocence Project chapters that are relatively new or not as robust as in other states where the organization helps prisoners navigate the arduous process of petitioning courts for DNA tests.

Prosecutors and judges can get in the way of post-conviction DNA testing:

In Oregon, the Innocence Project led the campaign for updating the DNA testing law, with cooperation from the state’s attorney general, district attorney, public defenders and the state’s crime lab, which formed a working group to hash out the new law and consider concerns. That type of cooperation is essential for states to better identify wrongful convictions, but it doesn’t always happen, Potkin said. While laws can remove some barriers to post-conviction appeals and improve DNA preservation standards, among the biggest challenges are prosecutor’s offices prone to fighting against the right to DNA testing and judges who tend to deny requests, she said. “Even if you have a law on the books, if you have prosecutors who will fight every application for testing and judges not willing to order testing over their objections, then the law is pointless,” Potkin said. She suggested that the prevalence of strong public defender systems and robust Innocence Project chapters in states could help mitigate some of the institutional resistance to post-conviction DNA testing.

Scale is one potential piece of the puzzle:

The 13 states are some of the least populated in the country, and collectively hold only about 6 percent of state inmates. Most inmates aren’t serving time for violent crimes like murder or rape in which DNA evidence might shed new light on their convictions. Despite those numbers, Potkin rejects the premise “that there is nobody in these 13 states whose innocence can be proven through DNA testing.” She finds it hard to believe that the 13 states would be free of some of the leading causes of wrongful convictions, including unreliable eyewitness identification, false confessions, prosecutors withholding exculpatory evidence, ineffective counsel and bogus testimony from informants. “It just doesn’t seem possible,” she said.

The proliferation of DNA technology might mean fewer wrongful convictions than in the past:

Veteran prosecutor Kristine Hamann, executive director of the Prosecutors Center for Excellence and co-chair of the National District Attorneys Association’s Best Practices Committee, said that the proliferation of DNA testing by law enforcement means that today many DNA evidence rules out would-be defendants before trial. Hamann, also a former New York inspector general with past stints as a federal prosecutor, argues that fewer wrongful convictions are occurring, and says that many of the DNA-based exonerations from the past were lower hanging fruit. However, what does that mean for the scores of people imprisoned before widespread DNA testing who are still fighting to prove their innocence in Iowa and other states? “There needs to be a reasonable standard for accessing testing, and making sure incarcerated people have the tools to do so,” Potkin said."

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