STUDIES: What Percent of Convictions Are Mistaken?
In June, the National Institute of Justice released the results of a study to determine how often modern DNA testing of evidence from older cases confirms the original conviction. The study, conducted by the Urban Institute in Washington, D.C, tested DNA evidence that had been retained in homicide and sexual assault convictions that occurred between 1973 and 1987 in Virginia. Among the homicides, there were not enough cases in which DNA would be determinative of guilt to make statistically reliable conclusions about mistakes. In cases of sexual assault, DNA testing revealed that in 8-15% of the convictions, the convicted offenders were eliminated as the source of questioned evidence and that elimination was supportive of an exoneration. The report concluded, “Even our most conservative estimate suggests that 8 percent (or more) of sexual assault convictions in a 15-year period may have been wrongful. That means hundreds, if not more than a thousand, convicted offenders may have been wrongfully convicted. That also means hundreds (if not more) victims have not received the just result, as previously believed. Therefore, whether the true rate of potential wrongful conviction is 8 percent or 15 percent in sexual assaults in Virginia between 1973 and 1987 is not as important as the finding that these results require a strong and coordinated policy response."
Researchers found that DNA testing in two-thirds of the cases yielded “indeterminate” results, meaning the test was not sufficient to determine if the DNA came from the convicted individual. Researchers also noted that there might be other evidence, not in the forensic file of the case, that could still point to the guilt of the defendant, though they concluded this would be "relatively rare."
(J. Roman, K. Walsh, et al., "Post-Conviction DNA Testing and Wrongful Conviction," Urban Institute Justice Policy Center, June 2012; posted by DPIC July 2, 2012). See Innocence. Listen to DPIC's podcast on Innocence.