Last week, police announced they had arrested Bryan Kohberger in the brutal murders of four University of Idaho students. After a weekslong investigation, authorities zeroed in on Kohberger in part by comparing DNA found at the crime scene with DNA of a relative of Kohberger apparently obtained from the family’s home trash.
With only limited information available, it is far too early to judge the strength of the prosecution’s case. But the role DNA has already played has drawn renewed attention to broader issues around the uses and limits of this technology — and scientific evidence more generally — in the criminal legal system.
At its best, DNA testing can tell you whose genes were found in a particular location — but it can’t tell you how they got there.
The approach used to identify Kohberger is just one of several recent developments in DNA analysis that have transformed the way law enforcement investigates crimes. While these advancements present tremendous potential for law enforcement and criminal investigations, burgeoning new areas of forensic analysis also have shortcomings that underscore the need to proceed with caution and recognize that not all technological advancements are foolproof.
Over the past decade, recognition of the deep flaws in many common forensic techniques — like blood spatter, hair or bite mark analysis — has grown, even as pop culture continues to depict often-questionable forensic methods as infallible determinants of scientific truth.
In a justice system rife with flawed science, DNA evidence is rightly viewed as the gold standard for forensics: After DNA evidence was first introduced in the criminal justice system in the late 1980s, scientists spent years developing and evaluating protocols for comparing individual DNA samples, which have been repeatedly found to produce consistent, reliable results.
This has enabled investigators to crack decades-old cold cases and contributed to the exonerations of at least 568 innocent people. But while the value of DNA analysis in criminal investigations is now widely recognized, the limits and pitfalls of DNA testing technology and other often less credible sciences are not sufficiently understood among prosecutors, law enforcement and the public.
The criminal legal system is increasingly relying on innovative new techniques for analyzing genetic material. As such, it’s important that we build a greater understanding among both the criminal justice field and the public of how DNA is used, what it can tell us definitively and what it cannot. That’s particularly critical in high-profile cases in which pretrial publicity around purportedly definitive “scientific results” can harden thinking and strip defendants of their right to a fair and unbiased jury.
DNA analysis was a revolution in forensics, but like every form of scientific evidence it has the potential for error. At its best, DNA testing can tell you whose genes were found in a particular location — but it can’t tell you how they got there. An innocent person’s DNA might be found at a crime scene because they brushed against the victim on the street, used the same doorknob as the perpetrator or dropped a cigarette butt nearby.
Recent technological advancements allowing scientists to analyze increasingly small or contaminated DNA samples further complicate the process. While these new techniques can yield valuable insights, they require more subjective judgment and thus are far more likely to produce false matches than traditional testing. DNA samples are also sometimes mishandled or tampered with, compromising results.
DNA often yields powerful insights, in particular when used in ruling out individuals: Analysts can identify inconsistencies between two DNA samples that make it nearly impossible for both samples to have come from the same person. But because the DNA of any two people is more than 99% identical, it is far harder to say with total certainty that two DNA samples must have come from the same person, particularly when dealing with incomplete or tainted samples.
And while the absence of a suspect’s DNA from the crime scene can, in some cases, provide strong evidence that they were not present or implicate another suspect, the presence of their DNA is not always evidence of guilt. As such, reliance on DNA alone in investigations, absent other corroborating facts, can lead to tunnel vision and confirmation bias, in which law enforcement and prosecutors subconsciously discount evidence that points away from their chosen suspect.
To guard against mistakes, law enforcement and forensic scientists must set strict protocols for collecting and handling genetic material to ensure samples aren’t contaminated or degraded. Prosecutors should be educated on the developments in, and limitations of, DNA analysis and other forensic techniques, and defense attorneys should always be provided with equal access to evidence and testing, allowing them to probe potential errors. Furthermore, forensic evidence should rarely, if ever, be the sole basis for arrest or prosecution — and the public should keep that in mind as it absorbs information in high-profile cases involving DNA.
The growing use of forensic genetic genealogy — the practice of comparing crime scene DNA with existing DNA databases to identify close relatives of the perpetrators — raises additional concerns. While promising in its ability to help solve cases, the technique has enabled law enforcement to gain access to the DNA of nonsuspects.
Furthermore, misuse of this information could carry profound consequences, and it’s crucial that we protect against abuses. Because Black and Hispanic people are significantly overrepresented in law enforcement databases, they are also disproportionately likely to be implicated by this genealogical analysis, potentially exacerbating racial disparities throughout the system.
In 2021, the Innocence Project worked with Maryland lawmakers to address these risks by passing the first law regulating the use of forensic genetic genealogy. The bill limits the use of these techniques to only the most serious violent crimes or cases that present immediate public safety risks. It also requires that law enforcement obtain informed consent before analyzing the DNA of nonsuspects (like family members), unless doing so would compromise the investigation.
The protections in the Maryland law should be replicated nationwide, especially as we continue to see law enforcement agencies exploit private health information — like DNA collected from newborn genetic screenings or examinations of rape victims — to identify crime suspects.
We also need protections against misuse of other forensic advancements. To that end, district attorneys should have a point person on forensics within their offices who is versed on these issues, and advocate for state forensics science commissions (like that created in Texas) to ensure that discredited sciences don’t form the basis for arrests, prosecutions and convictions.
We will have to wait to find out what the investigation into the unthinkable murders of four Idaho students ultimately yields. But while we struggle to confront the senseless horror of these killings, and commend the efforts of law enforcement to solve the case, we should also remember that each high-profile incident presents an opportunity to push for better, more rigorous standards. Victims and survivors of crime and the broader community, as well as individuals charged and arrested, deserve no less.