DNA and Other Forensic Evidence: Reliable or Fallible?

Is DNA and other forensic evidence reliable?Is forensic evidence, like DNA, always reliable? The answer may surprise you. My guest today is a fellow research junkie. I’ll let her explain why. Please welcome New York Times E-book Bestselling Author Debbi Mack to Murder Blog.

It’s an honor to be here today as Sue’s guest blogger. There are a few things I need to make clear from the outset. I am a lawyer, but no longer in practice. A practicing attorney needs many skills. Crime scene investigation is not necessarily one of them.

If I’d been an attorney who handled numerous criminal cases, I’d probably have more insight into police procedure than I do. But as you probably know, lawyers come with different fields of expertise. In my case, those fields included Social Security disability law, environmental law (I worked with the pesticides and toxics programs at EPA), plus zoning & land use law. I worked in each of these areas for a period of time before I decided to go into business for myself. That’s when the real learning started—when I opened my own office.

As a general practitioner, I did what my peers called “door law”, which was pretty much as it sounds—we handled whatever cases came through the door. Those could include a wide variety of legal matters, from wills to divorces to minor criminal cases. And not even once did I take on a client accused of murder.

But what all attorneys must do—and are, in fact, trained to do—is conduct careful research, read closely, and think like a lawyer. And since I write mysteries about a lawyer who investigates murders connected to her cases, I tend to research and read about the subject of criminal investigation a lot. Not to the point where my protagonist would be a CSI specialist, but at least to the point where she would know where to poke holes in a prosecutor’s case.

In other words, a defense lawyer needs to know enough about police procedure to be able to attack the veracity or trustworthiness of the prosecutor’s evidence. Because in the United States, everyone really is (theoretically) innocent until proven guilty beyond a reasonable doubt. That’s the highest standard of proof.

Now, setting aside everything I could say about how wrong it is to try a person in the media (or, these days, by social media)—which I could easily rant about at length—I’ll discuss why forensic and DNA evidence are not always conclusive proof of guilt.

Several examples of people wrongly convicted based on forensic evidence.

First, from July 2016 issue of National Geographic, here’s an excerpt from “Beyond Reasonable Doubt”:

“[O]ver the past decade or so, it’s become apparent that many forensic methodologies offer far less certitude than TV dramas suggest. And when forensic evidence is oversold in court, innocent people go to jail, or worse.”

Kirk Odom’s case is one example. He was prosecuted and convicted of rape based on an FBI analyst’s testimony that Odom’s hair was “microscopically indistinguishable” from a single hair found on the rape victim’s nightgown. Odom spent more than 22 years in prison and eight on parole as a sex offender before the public defender’s office dug up new evidence proving his innocence.Is DNA reliable? Guest post by Debbi Mack

Odom was lucky compared to Cameron Todd Willingham, who in 1992 was accused of setting fire to his house in Texas—a fire that killed his three daughters. Fire investigators interpreted the char patterns and what appeared to be multiple ignition points as evidence of arson. In 2011, the state of Texas found that the interpretation of that evidence was fatally flawed. Unfortunately, it was too late for Willingham, who’d been executed seven years earlier.

Then there was the case of Oregon attorney Brandon Mayfield, who was arrested by the FBI at his law office in May 2004. The agents were less than clear about why they were arresting him and he had to read the arrest warrant with his hands cuffed behind him. Apparently, his fingerprints had turned up in a computer search and were thought to match those on a plastic bag containing material used in the Madrid terrorist bombings that killed 191 people. Fortunately for Mayfield, Spanish authorities didn’t agree and they eventually found another match to the prints.

According to the article, “What all these stories have in common is their reliance on methods and interpretations that involve more craft than science. The power of hair analysis, for instance, has been vastly overstated. The FBI admits that its analysts have made erroneous statements in more than 90 percent of the microscopic-hair-comparison cases it has reviewed. …

“DNA evidence is hardly incontrovertible. Its value can be compromised by contamination from extraneous DNA anywhere along the chain from the crime scene to the laboratory where the sample is sequenced. … In April 2015 DNA analysis in the D.C. crime lab was suspended for 10 months and more than a hundred of its cases were reviewed, after an accreditation board found that analysts there were ‘not competent’ and were using ‘inadequate procedures.'”

Here’s more for you from a June 1, 2016 Scientific American article, “When DNA Implicates the Innocent”:

In December 2012, a homeless man named Lukis Anderson was charged with murdering Silicon Valley multimillionaire Raveesh Kumra, even though Anderson had an unimpeachable alibi: he’d been nearly comatose, hospitalized and under constant medical supervision the night of the murder. His legal defense team eventually learned that the paramedics who responded to Kumra’s medical emergency had treated Anderson three hours earlier and inadvertently transferred Anderson’s DNA to the crime scene.

According to this article, “Until recently, this type of DNA has been regarded as incontrovertible proof of direct contact. But a growing number of studies show that DNA does not always stay put. For example, a person who merely carries a cloth that had been wiped across someone else’s neck could then transfer that person’s DNA onto an object he or she never touched, according to a study published earlier this year in the International Journal of Legal Medicine. Similarly, Cynthia M. Cale, a masters candidate in human biology at the University of Indianapolis, recently reported in the Journal of Forensic Sciences that a person who uses a steak knife after shaking hands with another person transfers that person’s DNA onto the handle. In fact, in a fifth of the samples she collected, the person identified as the main contributor of DNA never touched the knife. Cale and her colleagues are among several groups now working to establish how easily and how quickly cells can be transferred—and how long they persist. …

“Just how often transferred DNA ends in a wrongful accusation is unknown. ‘Although clear cases appear to be quite uncommon, I think it’s probably more prevalent than we think,’ says Jennifer Friedman, a public defender in Los Angeles and DNA specialist. ‘The problem is that what we don’t see frequently is the ability to definitely prove that transfer occurred.'”

These findings underscore the potential dangers of assuming that science and technology are infallible. As always, when humans are involved, much can depend on chance and/or errors in judgment. Something to keep in mind if you ever serve on a jury in a capital case.

Is DNA reliable? Guest post by Debbi MackDebbi Mack is the New York Times ebook bestselling author of the Sam McRae mystery series. The first book in the series Identity Crisis was re-released in 2015 by WildBlue Press. Her young adult novel, Invisible Me, was chosen as the solo “Medalist Winner” in the Young Adult category of the New Apple Book Awards. Debbi’s short story anthology Five Uneasy Pieces includes her Derringer Award–nominated story “The Right to Remain Silent.” Her short stories have appeared in various other anthologies and publications. Her most recently published short stories are “Deadly Detour”, published as an ebook short, and “Jasmine”, which appears in Chesapeake Crimes: Homicidal Holidays. Her latest novel, The Planck Factor, is a science-based metathriller. 

Debbi hosts a podcast called The Crime Cafe, where she interviews mystery, suspense, and thriller authors. The podcast can be found on her website, iTunes, and StitcherDebbi is also a screenwriter and aspiring indie filmmaker, who blogs about movies at I Found it at the Movies. She’s releasing an I Found it at the Movies series of film essays and reviews, the first of which covers film noir. You can buy it at all the online retailers listed hereA former attorney, Debbi has also worked as a journalist, librarian, and freelance writer/researcher. She enjoys walking, cats, travel, ovies, music, baseball, and espresso. You can find Debbi online here: http://www.debbimack.com

Twitter: @debbimack My film review tweets: @FilmWoman

YouTube: http://www.youtube.com/c/DebbiMack

Facebook (author page): https://www.facebook.com/debbimackwriter

Facebook (The Crime Cafe) https://www.facebook.com/The-Crime-Cafe-383760808486685/

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About Sue Coletta

Member of Mystery Writers of America, Sisters in Crime, and International Thriller Writers, Sue Coletta is the bestselling, award-winning author of psychological thrillers and mysteries. OOTG Flash Fiction Offensive magazine published her flash fiction and her short stories are published in numerous anthologies and collections. InSinC Quarterly featured her forensic articles about Radiocarbon Dating and Skeletal Differences. In 2017, Feedspot awarded her Murder Blog as one of the Top 50 Crime Blogs on the net. Sue's also the communications manager for Forensic Science and the Serial Killer Project. As a way to help fellow crime writers, Sue created a team of crime experts (detectives, coroners, police captains, etc.) and founded #ACrimeChat on Twitter. She's also a proud member of the Kill Zone, where she blogs every other Monday.


  1. Great post. As a mystery writer, my creative mind is now churning with ideas for real/false accusations. Garry is so right, and also with the CSI effect, people unfortunately think not only is DNA analysis foolproof, but DNA results come back in hours instead of weeks.
    JHolmes, author recently posted…Fiction in real life settingsMy Profile

  2. An intriguing post. It’s staggering to learn of the horrible mistakes that have been made based on DNA and other forensic samples. My heart goes out to those wrongly judged and convicted. Thanks Debbie and Sue for shining a light on the flaws in the system.
    Mae Clair recently posted…The Monday Buzz from Story EmpireMy Profile

  3. Interesting read. DNA evidence is obviously an evolving, and not fixed, science.

  4. Wonderful post. One of those wrongful convictions would make a great story. Kind of an inside the prison exposure of the problem story. Seems like Tom Selek made a movie along those lines years ago.

  5. Great piece on the fallibilities of physical evidence, Debbi & Sue. Cross-contamination of DNA samples is a big problem and shows why each piece of evidence has to be assessed on its reliability factor. Somewhere I heard DNA being described as “dirty stuff”. It’s easily transferred and easy to misunderstand the actual implications. This is why every piece of evidence should have some corroborating backup that verifies reliability – not left to stand alone. That’s a recipe for false accusation let alone wrongful conviction.
    Garry Rodgers recently posted…THE BATTERED WOMAN SYNDROMEMy Profile

    • Thanks for weighing in, Garry! Excellent points about the need for corroborating evidence!

    • So true, Garry. In Massachusetts, the lab got shut down after the person who ran it got caught falsifying results (in the prosecution’s favor) for over ten years. Each case she worked on needed to be carefully scrutinized. Numerous criminals walked free, even criminals who had a lengthy rap-sheet. It’s now run by the State Police, which I understand is rare.

  6. It does seem that the more sensitive the DNA tests become (especially when the amount needed for testing is getting smaller and smaller), the more likely it becomes that the tests can become misleading by cross contamination. I would guess that right now there is more DNA on my own hands than just mine.

  7. This is really interesting! It’s useful, too, for crime writers – thanks, both. What I find fascinating (and troubling, too) is how much faith we tend to put in one or another sort of evidence, including DNA. It’s more reliable than some evidence, but it’s not foolproof.

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