DNA in the Court

While DNA profiling is considered one of the most powerful forensic tools, its role in the courtroom is not always straightforward.

The probative value of any piece of evidence is the probability of the evidence proving something important – in essence, the relative weight of each type of evidence. Courts generally assign high probative values to physical evidence. If without any sufficient explanation, there are several DNA matches to a suspect, including fingerprints at the crime scene and the victim’s blood on the suspect’s clothing, the DNA evidence would have a higher probative value and prove more compelling.

Further, there are many ways DNA can corroborate testimony, especially from a victim. In many cases, DNA evidence can help establish a link between “circumstantial connections” – it may be the final piece to convince the jury of a decision beyond reasonable doubt. In fact, the American Psychological Association states that “Many legal scholars fear that jurors expect DNA evidence in every case and that they follow DNA evidence blindly because of its perceived reliability,” expectations which they believe are only further exacerbated by media and popular culture.

Eyewitness testimonies have proved unreliable – and with significant consequences. Several people sentenced to life imprisonment and death row (who were later exonerated) were convicted solely due to eyewitness testimony, later revealed to be perjured. False confessions, which may arise in high-pressure situations and are usually coerced, “have been a contributing factor in approximately 25% of the DNA exoneration cases,” according to the Innocence Project. And according to research analysis by the American Psychological Association, “in the case of a defendant who had confessed, the prosecutor’s theories significantly attenuated the effect of otherwise compelling exculpatory DNA,” proving that the weight of DNA is directly dependent on other evidence presented in court.

However, DNA matches are also identifications beyond reasonable doubt themselves – they are inherently statistical probabilities, as a conclusion is made when profiles form the evidence DNA and the suspect’s DNA match “enough” times. Luckily, with the FBI having recently expanded the number of unique DNA markers searched in CODIS from 13 to 20, the “statistical power of forensic DNA testing” greatly increases, reducing errors in profile matching. (IP) However, there is still risk of human error, both in the collection and analysis of DNA. Thus, for DNA evidence presented to the court to have integrity, it must be carefully handled and analyzed, appropriately interpreted, and accurately reported to the jury and the judge.

Unfortunately, expert evidence is not always “expert.” Even when the DNA analysis is right, the interpretations are often misleading and presented to jurors differently by the prosecution and defense.

The prosecutor’s fallacy “occurs when investigators focus on the existence of the match, rather than the possibility that the match could be a coincidence” (Jrank) It is a statistical misrepresentation of Bayes Theorem, which governs conditional probability, or the probability of an event occurring giving something else has happened.

The prosecutor’s fallacy takes the answer to the question “Given the person is innocent, what is the chance that their DNA would be a match to the crime scene” and equates it to the answer to “Given the DNA is a match to the crime scene, what is the probability the person is innocent?”

These two conditional probabilities are different!

Suppose a suspect in a robbery matches the description of a person 1.75m tall, with rainbow hair and wings. In the specific city, let’s say there is a 1 in 100,000 chance of finding the person who fits this description. Given the person is innocent, there is a 1/100,000 chance that the person matches the description. The prosecutor’s fallacy will say that this is the same probability that the person is guilty – in other words, there is a 1/100,000 chance the person is innocent given the description was a match.

Why is this wrong? Let’s take the actual population of the city to be 1 million. This means a 1/100000 chance of someone matching the suspect’s description translates to 10 people out of the city population. Thus, given a match to the description, the probability the person is innocent is 9/10, or 90%. This is the true probability of the second question, and this is also known as the defender’s fallacy. This is also a fallacy as it ignores the necessary exclusion of certain of these 10 people as suspects – if the match wasn’t for a description but for DNA, and a baby’s DNA matched, the baby could not be ruled a possible suspect – as well as fails to account for all the other factors pointing to the defendant’s guilt.

This was used in the trial of OJ Simpson, where the defending lawyers argued that “the blood samples from the scene matching OJ’s (1 in 400 match rate) could also be matched against thousands of others across the LA population,” to establish that the DNA evidence was ineffective. (TDS) The Supreme Court also had to elucidate on this in their case McDaniel v. Brown.

While the prosecution targets the high precision and advantages of DNA testing, the defense underscores errors and unreliability in reaching conclusions through DNA analysis. The prosecution takes advantage of the fact that DNA is omnipresent, that it may be accurately analyzed much after its collection due to the fact that it does not decay. They also rely on general confidence in the unbiased performance of DNA testing by forensic and commercial laboratories. The defense often capitalizes instead on the risk of contamination of DNA samples due to faulty police procedures and laboratory work. Barry C. Scheck, one of the experts called in for the defense of O.J. Simpson in his murder trial, “pointed to research that showed that as many as one to four percent of the DNA matches produced by laboratories were in error.” Further, some argue that DNA matches are more likely to occur when considering samples drawn from subgroups where the individuals are already genetically similar, such as in “tightly knit immigrant or religious communities” or when suspects are related. (Jrank)

There is no doubt that DNA has played a significant role in court, but expert rephrasing of DNA match analysis could mislead a jury about a defendant’s guilt, and thus the true weight of DNA in court is extremely subjective, based largely on how the analysis is presented.

Sources:

https://www.apa.org/monitor/2009/05/jn

https://www.publicdefenders.nsw.gov.au/Pages/public_defenders_research/Papers%20by%20Public%20Defenders/public_defenders_dna_itsuse_andmisuse.aspx