New Book Explores DNA Evidence in Crime and Forensics

AMES, Iowa — Matt DeLisi, a renowned criminologist and scholar at Iowa State University, examined over 100 cold cases recently resolved by DNA evidence in his latest book. "What DNA Evidence Reveals About Criminals: Cold Case Criminology" demonstrates how new forensic technologies, investigative techniques and policies can provide justice to victims and closure for families.

"There's a whole forensic frontier of concepts that academic criminology isn't focusing on, and I think there's a story to tell about what these offenders are like. This book provides insights about both," says DeLisi.

Matt DeLisi, Distinguished Professor

Matt DeLisi has written more than 500 scholarly publications and over 30 books, including "Ted Bundy and the Unsolved Murder Epidemic: The Dark Figure of Crime" in 2023.

The Distinguished Professor and College of Liberal Arts and Sciences Dean's Professor focuses each chapter on a behavioral feature that is central to criminal offenders: opportunity, self-control, noncompliance, versatility, specialization, continuity, intermittency, severity and comorbidity.

DeLisi emphasizes that serial or repeat offenders, especially those who perpetrate sexual offenses, are highly opportunistic and have low self-control. They strike when a situation presents itself – a hitchhiker along an empty road, an unlocked door. The spontaneous randomness of victimization makes it difficult for investigators to narrow in on a suspect.

This is one of the reasons why DNA evidence from hair, fingerprints, saliva and other fluids is invaluable. DeLisi says it offers an opportunity to search through state and federal databases for a match and catch repeat offenders who commit the largest share of violent crime. In recent years, the development of investigative genetic genealogy has also opened doors to find perpetrators.

In 2019, 52 years after the sexual assault and murder of Rita Curran in Vermont, detectives sent a cigarette butt from the scene of the crime to Parabon NanoLabs. A genealogical investigator found very close matches to two relatives of William DeRoos, a neighbor who lived in Curran's apartment at the time. Investigators couldn't charge DeRoos; he died 15 years after Curran's murder. But reopening the case provided answers and resolution for Curran's family and community. DeLisi says DNA evidence also excludes and exonerates suspects and helps identify missing persons.

"The contemporary use of DNA evidence and associated technological methods to solve cases is nothing short of a revolution in the criminal justice system, one with broad aftershocks in the fields of law, law enforcement, corrections, genetics, and forensic genealogy," DeLisi writes in the book.

Closing more cold cases

Yet, DeLisi says much more can and should be done to resolve cold cases. The U.S. has an estimated 250,000 to 350,000 unsolved homicides and around 400,000 unprocessed sexual assault test kits. Only 7% of law enforcement organizations have a formal cold case unit, and most agencies don't have protocols for cold case investigations. DeLisi adds that some agencies are "reluctant to submit DNA testing to labs because they aren't able to return timely results or aren't accepting new forensic evidence."

More funding to clear the backlog of sexual assault test kits, develop dedicated cold case units and expand training could help, he says.

"States also have different regulations about when DNA samples can be collected and what crimes qualify for DNA collection. If we make state laws more expansive in terms of who they can collect DNA from, that increases the likelihood that they could connect to the FBI's Combined DNA Index System or a state genetic database," says DeLisi.

DeLisi also supports more widespread use of John Doe warrants, where the suspect is unknown but identified by DNA. Prosecutors file a warrant with 'John Doe' as the placeholder when the perpetrator is unknown to prevent the statute of limitations from expiring.

"Expanding the statute of limitations for severe crimes including attempted murder, kidnapping and sexual assault with elements of torture or other aggravating circumstances also makes sense," says DeLisi.

His book includes several examples of serial offenders who were convicted for violent but non-lethal crimes and later linked to homicides through DNA evidence.

DeLisi says using "clearance by exceptional means" in certain situations could resolve more cases, as well. Richard Cottingham, known as the Torso Killer, was serving multiple life sentences in New Jersey for killing and sexually assaulting 11 women in the 1960s and 70s. Investigators suspected Cottingham was responsible for more unsolved murders but couldn't link him to the cases.

"Cottingham couldn't be punished more. New Jersey is not a death penalty state, but the authorities in cases like his can use clearance by exceptional means and say, 'If you can provide corroborating information and evidence that would support an arrest that leads us to believe you did kill this person, you will acknowledge your involvement in it, but you're not going to be prosecuted for it.' It can help authorities find the body and give legal closure to families," says DeLisi.

In 2022, Cottingham confessed to five additional murders, including one that matched DNA evidence collected five decades prior.

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