You’ve been accused of a violent crime – and the police say that they found your DNA at the scene, so the prosecutor is promising a conviction.
So what? While DNA evidence has become a sort of “gold standard” for proving guilt or evidence in the popular mindset, the reality is often a lot more complicated. DNA isn’t nearly as infallible as popular courtroom dramas have led many to believe, and it is often successfully challenged for one or more of the following reasons:
Contamination
One of the biggest challenges with DNA evidence is contamination. Small amounts of “trace” DNA can be easily transferred from one person or object to another, often without anyone realizing it.
This means that DNA found at a crime scene may not necessarily belong to the perpetrator. For example, a suspect’s DNA could be present at a crime scene if they had previously visited the location or if their DNA was transferred through an intermediary – like a paramedic who treated a patient at one scene and then transferred their genetic material to a homicide scene later.
Lab error and misuse
Even in the best laboratories, human error can occur. Mistakes in labeling, sample handling or data interpretation can lead to incorrect results. Additionally, forensic lab techs may face pressure to produce results quickly, potentially leading to rushed or incomplete analyses – especially in high-stakes cases.
DNA evidence is also not immune to bias. Forensic scientists can be influenced by cognitive biases, where they may unconsciously interpret evidence in a way that supports their preconceived notions. Furthermore, the misuse of DNA evidence by overzealous prosecutors or investigators or lab techs has led to wrongful convictions.
Statistical interpretation
The statistical aspect of DNA evidence can also be misleading. DNA matching relies on probabilities, and while the chances of a random match can be very low, they are not zero.
Misunderstanding or misrepresenting these probabilities can lead to overconfidence in the evidence. For instance, a match probability of one in a million sounds conclusive, but with a large enough population, coincidental matches can and do occur – especially when DNA evidence has degraded over time or is only partially complete.
Don’t ever assume that your case is hopeless because a prosecutor says they have your DNA. An experienced defense can help you see the options.
