Your own DNA can be found at a crime scene even if you were never present

Ever since the emergence of DNA profiling in criminal investigations in the early 1980s, DNA forensics has revolutionised criminal justice. It has not only prosecuted the guilty, but also exonerated the wrongfully convicted, cementing it’s place in modern justice as the ‘gold standard’ in criminal prosecution. Experts have told us that the perpetrators of crimes will leave traces of DNA at the crime scene, and if found in small amounts, the evidence can be used to identify the perpetrator. Finding a suspect’s DNA at a crime scene, on the victim, or a piece of evidence informs police of the suspect’s presence at the scene of the crime, contact with the victim or a relevant piece of evidence.

However, the reliance on ostensibly infallible DNA evidence to prosecute the guilty carries significant implications that threaten to undermine, disrupt, and subvert the criminal justice system.Research conducted at the University of Indianapolis in the USA in 2016 suggests that the detection of DNA at a crime scene may not indicate presence or contact of that individual at all. This is because humans shed DNA continuously, leaving traces of our genetic identity practically everywhere we go. An average person may shed up to 50 million skin cells a day. Attorney Erin Murphy, author of Inside a Cell, has calculated that in under 2 minutes the average person sheds enough skin cells to cover an entire football field.

Forensic scientists profit from this “abandoned” DNA to solve crimes; DNA left behind on phones, handles, keyboards, cupboards, couches, kitchen utensils and numerous other objects, not to mention bodily fluids such as blood and semen. Scientists refer to this phenomenon as “secondary transfer DNA” that can be transferred through contact or touch.

Australian forensic scientist Roland van Oorschot was one of the few scientists who spearheaded the investigation of secondary transfer DNA. In one of his lab experiments, volunteers sat at a table and shared a jug of juice. After 20 minutes of socialising and sipping, DNA swabs were taken from the jug of juice, the chairs, the table, the juice glasses, and hands of the participants. Although the participants never actually came into direct physical contact with one another, 50 percent wound up with each other’s DNA on their hands. A third of the glasses bore the DNA of volunteers who did not touch or drink from them. Then, there was the foreign DNA- that is, DNA profiles that didn’t match any of the participants. It turned up on approximately half of the participants’ hands and table. The only viable explanation? The participants unintentionally introduced alien genes into the experiment. Perhaps from the business associate they shook hands with that very morning, the stranger they shared a door handle with to exit their apartment building, or from the thousands of strangers who used the same taxi at one point or another to travel to work.

Similarly, an individual detected by a match of DNA discovered at a crime scene may have never come into contact with the crime scene. However, modern day investigations do not account for the possibility that the DNA evidence procured from a crime scene may be DNA transferred by the innocent.

In 2016, Four Indianapolis scientists questioned the reliability of contemporary forensic lab technologies in the January issue of the Journal of Forensic Sciences, with a paper entitled “Could Secondary Transfer DNA Falsely Place Someone at the Scene of a Crime?” The study was inspired by a concern that arose at Strand Laboratory in Indianapolis, when routine contamination investigations began turning up DNA profiles of the lab worker’s children. The study, spearheaded by lead analyst Cynthia Cale of Strand Diagnostics, concluded that touch-transfer DNA could “falsely link someone to a crime” and that scientists relying on modern forensic technologies could “falsely conclude that DNA left on an object is a result of direct contact.” Their findings revealed that it is sometimes impossible to determine whether the tiny bits of DNA found at a crime were directly deposited at the crime scene, or indirectly made their way via a secondary source.

“The research highlights the need to eliminate ‘touch DNA’ from our vocabulary,” said Madison Earll, a Human Biology student who participated in the study, positing that the term is clearly “misleading” and “does not adequately explain all the possible ways that DNA can end up on an object.”

It is perhaps even more troublesome that prosecutors often have no qualms about presenting touch-transfer DNA as incontrovertible proof to juries; juries that may have little knowledge of the scientific properties and behaviour of secondary transfer DNA. Overcome with anxiety and fear of making the wrong choice based on circumstantial evidence, juries often opt for forensic and DNA evidence when determining criminal culpability. Thus, when prosecutors present touch-transfer DNA evidence to unaware juries, the jurors, influenced by conventional wisdom that DNA is inherently trustworthy, feel compelled to convict. It’s scary to think of the thousands of people that have been convicted worldwide as a result of precarious DNA evidence.

Of course, these concerns about the reliability of DNA evidence traverse the boundaries of the theoretical. In 2007, American woman Amanda Knox was charged with the homicide of her U.K housemate Meredith Kercher in Perugia, Italy. The Italian authorities had accused a local young man named Rudy Guede with sexually assaulting and killing Kercher. The evidence against him was a smoking gun-palm prints, fingerprints, and his DNA on the victim and throughout her room- and he was eventually found guilty. Yet an Italian jury also convicted Amanda Knox and her boyfriend Raffaele Sollecito for the murder, based on a minuscule amount of secondary transfer DNA. Knox’s DNA, and that of the victim were found on a kitchen knife in the home of Knox’s boyfriend. Since the victim was never in the co-conspirators residence, the prosecution insisted the only way the victim’s DNA could have made it to the knife was through direct contact- the murder. As such, an Italian jury found Knox guilty of murder, and her boyfriend guilty as a co-conspirator. Only in 2015 with greater understanding of the scientific properties of transfer DNA, was Knox exonerated for the crime.

On January 21st 2016, Daniel Holtzclaw, a former Oklahoma police officer, was sentenced to 263 years in prison after a jury wrongfully convicted him of sexually assaulting seven women and even one teenager, despite a lack of physical evidence, numerous inconsistencies in accuser’s testimony, and a failure of the data of the patrol vehicles Automatic Vehicle Location (AVL) in matching crucial accusations. The only forensic finding utilised by the prosecution during the trial was the DNA of a 17 year old girl found on the officer’s uniform pants. The young girl accused the officer of putting his hands in her purse, his hands plausibly coming into contact with a plethora of her DNA, before patting her down, and then brutally raping her. Indeed, the prosecution’s own DNA analyst acknowledged that the evidence was consistent with the properties and behaviours of secondary transfer DNA; an unaccounted-for and unknown male’s DNA had also been found on the pants together with the female’s DNA. Yet the prosecution definitively insisted that the transfer DNA found on the officer’s pants was irrefutable proof of sexual contact between Holtzclaw and the victim. In an egregious miscarriage of justice, the officer remains imprisoned to this very day.

As of now, the criminal justice system’s reliance on secondary transfer DNA evidence remains to be a double edged sword. Anytime we touch a public surface, anytime we shake hands with a stranger, anytime we use a handle to walk into a cafe, we remain fair game for criminal suspicion based on this modern forensics.

Moving towards the future, law enforcement must reduce reliance on DNA evidence to determine criminal accountability while investigating a crime. It should be commonplace for judges to sustain defence motions for exclusion of secondary transfer DNA on the grounds that the DNA may prove to be more prejudicial than probative, misleading unaware juries into making false convictions; convictions that destroy lives, convictions that threaten to destabilise the courts and our criminal justice system. DNA is no magic bullet, rather, it needs to be handled with scrutiny just like any other piece of evidence.