Timothy Durham was convicted of rape when a test showed his DNA genotype matched the DNA recovered from the crime scene. Eye witnesses testified that Durham was in a different state at the time of the incident, but he wasn’t exonerated from his 3000 year sentence until follow up DNA analysis showed that the preliminary forensic analysis used in the trial was misinterpreted (Thomas et al.). DNA testing serves an invaluable part in our criminal justice system, but it comes with complications and bias that can lead to wrongful convictions and misinformed juries.
Background:
Forensic genetics aims to identify the origin of a biological sample. It can be traced back to Karl Landsteiner who discovered human ABO blood groups. With blood groups, suspects could be eliminated based on their blood type. For example, if a suspect had type A blood and there was type B blood left at the scene by the perpetrator, the investigators could rule out the suspect. Unfortunately, the reverse didn’t work. If the suspect and perpetrator both had the same blood type, it can’t be concluded that the suspect was guilty. Blood identifiers evolved into 8 distinct categories which improved the ability to eliminate suspects, but could only conclude that (if the system matched) it might be the perpetrator or this person might be related to the perpetrator (Jobling and Gill).
The field evolved in the 1980s with the discovery of Minisatellites, lines of repetitive DNA that don’t encode proteins. Minisatellite patterns in DNA create what we know today as the DNA fingerprint, and it has a much higher accuracy rate than previous methods (Augustyn et al.). Around the same time, a breakthrough for victims of sexual assault was discovered. In the case of assault, bodily fluids of both the victim and the assaulter can get mixed. A process called differential lysis was created which could selectively increase the concentration of sperm in a fluid sample thereby allowing for accurate DNA testing (Jobling and Gill).
Forensics evolved to use single-locus probe testing after several years because of its ease of interpretation. It looked at specific minisatellites that had high variability between subjects. This was the first DNA-based method to be used in a criminal investigation. Two murder homicides from several years apart were connected using DNA evidence. The evidence was then used to exonerate an innocent man, and the guilty party was found when he tried to evade mandatory DNA testing for all men in the small town. His guilt was then reinforced when the DNA profiles matched, and he gave a confession (Jobling and Gill).
Finding DNA at a crime scene doesn’t guarantee that you’ll find a murderer. There are issues with non-pure samples and limited samples that make DNA work difficult, and accreditation of facilities and new techniques and take time (Jobling and Gill).
Subjectivity of DNA Interpretation:
In a 2009 publication, the National Academy of Science discusses the possibility for subjective influence in lab based forensics. The interpretation bias happens in complex cases where, for example, there could be a mixture of DNA. A study published in 2011 looked specifically at this issue.
A gang rape happened in Georgia, and the mixture in DNA from the case was used to prosecute one suspect after another suspect took a plea bargain to testify against him. With DNA mixtures, there are three possible conclusions: suspect excluded, suspect cannot be excluded, and inconclusive. If the suspect is excluded, it means that there’s little to no possibility that his DNA is part of the sample. If the suspect cannot be excluded, it means that there’s a possibility that his DNA is part of the sample. In the trial, the DNA examiners who knew the context of the case concluded that the suspect being prosecuted could not be excluded.
The 2011 study gave the DNA evidence from this case and the DNA profiles of the suspects to 17 forensic examiners. These examiners had no context of the case, and they all worked independently. Of the 17, only 1 came to the same conclusion (suspect cannot be excluded) as the examiners who knew the context of the case. Twelve of the 17 concluded that the suspect who was prosecuted and convicted had no relation to the DNA found in the sample. Not only does this show that subjectivity occurs due to the context of the case, but the 17 examiners came up with different findings based on their past experiences meaning that not all DNA analysis is objective. Some organizations like the
Scientific Working Group on DNA Analysis Methods say that quantitative statistical evidence should be presented with every categorical conclusion, but the US doesn’t require this (Dror and Hampikian).
False Positives:
There are two categories that cause wrongful identification in a DNA report: coincidental matches and false positives. Coincidental matches are DNA matches between two unrelated people. The probability of these matches is about 1 to 500,000 for white individuals and about 1 to 350,000 for black individuals. This small probability of coincidental matches makes people feel confident in DNA evidence. False positives, on the other hand, are matches between DNA and test DNA that result from issues in the forensics lab. There have been multiple cases of forensics labs accidentally switching victim and suspected rapist DNA which lead to convictions that were later overturned when the lab realized the error. There are also cases of misreading or misunderstanding a sample. In a case from 1993, a man was convicted of rape because someone interpreted a mixed sample of victim and perpetrator DNA as an individual sample. The conviction was again overturned. To prevent false convictions, retesting DNA is allowed, but it’s not an efficient solution. Many people are pushing for statistical information on the probability of false positives, so that the jurors can make accurate judgements based on DNA evidence (Thomas et al.).
Conclusion:
DNA analysis is a useful tool for the criminal justice system, but subjectivity can be an issue in forensic testing. To combat wrongful convictions, there are organizations like
The Innocence Project who use DNA testing to free the innocent, but reformed forensic testing standards should be put in place to prevent subjectivity and to ensure juries are correctly informed during trials.
https://www.innocenceproject.org/
Citation:
Augustyn , Adam, et al. “DNA Finerprinting.”
Encyclopedia Britannica , 26 Apr. 2018, www.britannica.com/science/DNA-fingerprinting#ref153186.
Dror, Itiel E., and Greg Hampikian. “Subjectivity and Bias in Forensic DNA Mixture Interpretation.”
Science & Justice, vol. 51, no. 4, 2011, pp. 204–208., doi:10.1016/j.scijus.2011.08.004.
Jobling, Mark A., and Peter Gill. “Encoded Evidence: DNA in Forensic Analysis.”
Nature Reviews: Genetics, vol. 5, no. 10, Oct. 2004, pp. 739–751., doi:10.1038/nrg1455.
Thompson, William C, et al. “How the Probability of a False Positive Affects the Value of DNA Evidence.”
Journal of Forensic Sciences, vol. 48, no. 1, Jan. 2003, pp. 1–8., doi:10.1520/jfs2001171.
I have been looking forward to seeing the movie Contagion since it came out last year and I finally got to see it last night. I am a big fan of outbreak/apocalypse/zombie-type movies and, since I’m an immunologist, I thought this would be right up my alley. Boy, was I wrong. The problem with the movie wasn’t with the premise (a new flu-like virus makes the jump from an animal reservoir to humans and starts spreading rapidly), the casting (Gwyneth Paltrow, Matt Damon, Jude Law, Kate Winslet, Laurence Fishburne), or the science (mostly spot on). The problem was that the movie was way too realistic.
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marcsmerlin
It’s been almost a year since I saw “Contagion” and I still marvel at its achievement as a science film. It is a daring movie in this regard, with director Stephen Soderbergh giving us a film that is brazenly proud of science and scientists.
There is a tendency to asses the science in a science film according to the accuracy of the information that it presents. Fair enough. The history of science – not to mention science fiction – films is littered with such many laughable errors. But allowance should be made to shortcuts made in service to the narrative; movies compress an enormous amount of story-telling into a two-hour package. That Kate Winslet’s character, a CDC EIS officer, explains basic epidemiology to a group of state health department officials, is a forgivable expository device: her intended audience is not the Minnesota Department of Health, it is us.
The only glaring scientific error that I spotted in “Contagion” had to so with the fact that the vaccine developed to defeat the raging MEV-1 virus confers immunity immediately. Given the nature of the human immune system this is hardly believable – for example seasonal flu vaccines take a couple of weeks before they provide full protection – but given the human interest in seeing the young lovers of a significant subplot united at last, this concession to the storyline must be graciously forgiven.
The triumph of “Contagion” is not so much that it gets its medical science facts right – and, for the most part, it does – but that it dares to show science as a complex interplay of individually challenging activities. Here director Soderbergh has partitioned the science story into three significant interacting systems. There is the detective story of the determining epidemiology of the new disease; there is the action story of confronting the imminent public health challenge, containing the spread of the virus while tending to the sick and the dying; and there is the story of discovery leading to the development of a vaccine that will halt the epidemic.
These three threads are intertwined to form the core plot of “Contagion” and each of them is assigned a heroine – brilliant, selfless and driven – preternaturally able to stay on task when everyone about them are losing their heads. They make Iron-man and Spider-man and Bat-man look like a chumps, as they confront death without bluster or fanfare, their heroism emanating from both an intellectual longing and from an unassuming compassion for their fellow human beings. As I said in my own blog on this movie (http://bit.ly/OxqfYs), collectively these three woman create “a calm, efficient working center at the eye of the swirling global disease storm.”
What is astounding with “Contagion” is that a big-budget movie would explore the complexity of the scientific enterprise with such unprecedented depth. It is a testimony to Soderbergh’s standing and independence as a director that this movie got made at all. I doubt that we will see the likes of it again anytime soon. We can only hope.