DNA Evidence Is Not Foolproof

Believing it blindly could put the wrong people in jail

A Customs and Border Patrol agent looks at a product's DNA to determine its origin.

CBP via Wikimedia Commons

Since it made its courtroom debut in the mid-1980s, DNA evidence has been integral to thousands of cases (including, famously, the OJ Simpson murder trial). Juries and lawyers alike generally consider DNA evidence to be extremely reliable—a 2005 Gallup poll found that 58 percent of people considered it to be "very reliable." But in reality, DNA evidence is much less reliable and objective than most people think. A story published yesterday by Frontline maps out just how DNA evidence works and how it can lead juries astray.

DNA evidence can be unequivocal under ideal conditions: when officials have a large quantity of a suspect’s well-preserved genes, when it’s clear how that DNA arrived at the crime scene, and when the labs sequencing the sample don’t make any mistakes. But there are very few cases in which all of these conditions are met. That means that most DNA evidence presented in courtrooms has some degree of ambiguity to it, which juries may not realize.

As Frontline explains, here's how DNA to be used as evidence:

…Crime labs don’t exactly map the human genome. Instead, they typically focus on 13 places, or loci, plus a 14th that expresses gender. Each loci is home to two alleles, one inherited from each parent. On an electropherogram, these alleles show up as spikes, and vary from person to person. We usually share half or more of our markers with close relatives, and often share several with complete strangers. But by the FBI’s statistics, the probability of sharing all 13 loci with someone you’re not related to is lower than 1 in a trillion.

To choose a suspect, investigators try to match the sample loci and the suspect’s loci as closely as possible. But often, crime scene samples are imperfect and the DNA breaks down, so the loci are weak. That makes finding a 100 percent match very challenging.

Under certain circumstances, those cells could end up in places we ourselves have never been.

But even if that match exists, there’s no way to tell how the crime scene DNA sample got there in the first place—scientists still don’t know how DNA can travel. Many of our body’s cells contain our DNA and we shed them wherever we go. Under certain circumstances, those cells could end up in places we ourselves have never been. Plus, they don’t have a timestamp on them, so DNA deposited on a crime scene six months before a crime occurred may end up as part of the case.

To reduce error and the likelihood of contamination, labs prefer a lot of crime scene samples with the suspect’s DNA. Soon technology will likely enable labs to sequence DNA off a single cell. But this may not mean that the evidence will be easier to interpret, especially because there’s already a lot of variation in how the labs process DNA samples, changing the conclusions that investigators can draw from the evidence.

Overall, the ambiguity of DNA testing methods means it is hard to know precisely in which and how cases DNA may have led juries and judges astray. But looking forward, If more people know that DNA evidence is not nearly as reliably black and white as it may seem, then maybe fewer suspects will end up wrongfully incarcerated.