DNA's Role in Uncovering Wrongful ConvictionsJun 21, 2012
NIJ has released a report based on retrospective DNA testing of physical evidence that had been retained in homicide and sexual assault convictions that occurred during a 15-year pre-DNA time period in Virginia. The goal of the study, conducted by the Urban Institute, was to determine if modern DNA testing of probative evidence would confirm the original conviction.
About the Study and Findings
In 2008, NIJ funded the Urban Institute to evaluate DNA testing results from a sample of 634 homicide and sexual assault cases (715 convicted offenders) that occurred in Virginia between 1973 and 1987. The goal of the study was to determine what proportion of the people convicted in those cases might be exonerated if evidence that was retained in the cases was DNA tested.
The researchers sorted the DNA test results into four categories:
- Indeterminate — DNA testing could not determine whether the convicted person was the source of the DNA on the evidence.
- Inculpatory — DNA testing confirmed that the person convicted was the source of the DNA.
- Exculpatory but insufficient to support exoneration — DNA testing eliminated the convicted person as the source of the DNA, but elimination was not sufficient to support potential exoneration without additional probative evidence.
- Exculpatory and supportive of exoneration — DNA testing eliminated the convicted person as the source of the DNA, and that elimination supported potential exoneration.
For a variety of reasons — including the age of the biological evidence — DNA testing in two-thirds of the cases yielded "indeterminate" results, meaning that DNA testing was not sufficient to determine whether the convicted person was the source of the DNA.
Among homicide cases, the high number of indeterminate results meant that the sample size was too small for the researchers to make any meaningful conclusions regarding a potential wrongful conviction rate.
More than half of the sexual assault convictions, however, yielded determinate results: In 227 of 422 convictions for sexual assault, DNA testing was sufficient to determine whether or not the convicted person was the source of the DNA.
In 33 of the sexual assault convictions, DNA testing of probative evidence eliminated the person convicted, and that elimination supported exoneration. The 33 convictions are 8 percent of the total number of sexual assault convictions in the study (422) and 15 percent of the convictions in the study that yielded determinate results (227).
Study Context and Limitations
It is important to keep in mind that the data and the study's findings have a number of limitations. The study looked at a single state, covered a single 15-year time period and only addressed cases in which forensic evidence had been retained. Additionally, the study did not address non-DNA factors that might have been relevant to the original convictions, such as the method of conviction, type of defense attorney (appointed or retained), and victim and eyewitness identification.
Read the full report, Post-Conviction DNA Testing and Wrongful Conviction.
For more information about NIJ's initiatives in postconviction DNA testing, see Postconviction DNA Testing Is at Core of Major NIJ Initiatives from the NIJ Journal.