Study examines impact of DNA evidence in sexual assault prosecutions

CHAMPAIGN, IL – When DNA evidence is available and matches suspects in sexual assault cases, prosecutors are more likely to prosecute and the odds of a conviction are more than nine times greater than in cases without biological evidence , according to a new study.

In a project Funded by the National Institute of Justice, researchers examined the role of DNA evidence in more than 100 sexual assault cases accepted for prosecution in a metropolitan jurisdiction between 2005 and 2010.

They found that DNA evidence had a dramatic relationship with the progression of cases through the criminal justice system and conviction rates. Nearly 75% of cases in which the DNA profile matched the suspect resulted in a guilty plea or trial, compared with less than a third of cases without a lab report.

“We did what we thought was the deepest and most incisive research on how DNA is actually used and how it relates to the outcome of sexual assault prosecutions,” said first author Ted Cross, a specialist in senior research fellow and professor of social work at the University. of Illinois Urbana-Champaign. “What we’ve found is that this evidence really matters.”

Megan Alderden, Professor and Chair of Criminology at DePaul University; and Laura Siller and Maja Vlajnic, then both PhD students at Northeastern University, co-authored the study. Published in the journal Violence against womenthe research sheds light on some of the complexities of using DNA evidence in sexual assault prosecutions.

The team said their findings underscore the importance of quality forensic examinations, investments in DNA analysis and training for prosecutors in the effective use of DNA evidence.

“Our research benefited from a statewide system in Massachusetts that tracks forensic examination data in sexual assault cases,” Cross said. “Each vendor who performs a forensic examination faxes a report to a state agency. This created a dataset that allowed us to link prosecutors’ cases to actual crime lab results, determine what those results were, and what actions prosecutors took.

Siller and Vlajnic also spent hundreds of hours going through prosecutors’ paper files and coding data that was linked to information in the state database.

Despite public misperceptions that biological evidence is ubiquitous in criminal justice cases, DNA evidence is not for granted, Cross said. Prosecutors must actively seek it out, and they are selective about which cases they take extra steps to obtain it.

“The DNA test is a separate test from standard lab tests, and not all cases undergo this test,” Cross said. “Crime labs and police departments are overwhelmed. Sometimes evidence kits don’t make it to the crime lab. And even if they do, they are not necessarily tested.

To match DNA with a suspect, prosecutors often need to obtain a court order for police to bring the suspect in for a sample, usually with a mouth swab.

“In this regard, DNA evidence can be both the cause and the result of the decision to prosecute a case, and prosecutors need to be aware of this complex relationship,” Cross said.

In the majority of the 106 cases investigated, a crime lab report was available but the sample did not match the suspect, the researchers found. DNA matches to suspects have occurred in about 25% of cases, according to lab reports or prosecution records.

Matching a DNA profile to an unknown assailant whose data is stored in the FBI’s DNA database occurs in a relatively small number of cases, Cross said.

Looking at how often prosecutors used multiple types of evidence, the researchers found that physical evidence from the crime scene and evidence of non-genital injuries were used in more than a third of cases. Evidence on DNA matches to suspects was used less frequently, presented in just over 20% of cases.

In a related study by the team published in the Journal of Interpersonal Violence in 2021, prosecutors said biological evidence such as DNA was important in preparing for their trial because of a potential “CSI effect” – a reference to the TV crime drama that suggests jurors expect that biological evidence is presented.

“Some prosecutors believe that juries expect forensic evidence even when it is not reasonable to obtain it and it is not strictly probative, that is, it does not help not logically prove the case,” Cross said. “Either way, prosecutors felt that juries needed it, wanted it, and expected it.”

Similarly, prosecutors said victims’ willingness to undergo a forensic examination — which can be long, uncomfortable and emotionally difficult, Alderden said — boosts their credibility with jurors and indicates that victims and prosecutors are taking the case seriously.

DNA evidence helps establish that the suspect had sexual contact with the victim – even if the suspect claims the act was consensual – and contradicts claims that the allegations are fabricated.

However, widespread public awareness and expectations of juries regarding forensic evidence can make it difficult to prosecute sexual assault cases without a DNA match, the researchers said.

There are many circumstances beyond victims’ control when biological evidence cannot be obtained although an assault has occurred, such as when an attacker uses a condom or situations that prevent law enforcement from investigating. obtain a sample from a suspect, according to the study.

“Given the amount of trauma suffered by victims, it is important that criminal justice practitioners and jurors do not jump to the conclusion that just because there is no DNA evidence, there is an indication that the victim is not telling the truth,” Alderden said. “DNA and other biological evidence just isn’t always available in these cases.”

– This press release originally appeared on the University of Illinois at Urbana-Champaign website.

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