Issues in Interdisciplinarity 2018-19/Reliability of Legal Evidence

This chapter will explore the benefits and drawbacks of two types of legal evidence, eyewitness testimony and DNA, with reference to the disciplines of psychology and forensic science. It will evaluate the reliability of these two types of evidence, by examining their values and implications in the world of law.

Reliability in the Context of Different DisciplinesEdit

In law, reliability of evidence is the degree to which the examiner is able to rely upon it in coming to a decision.[1] However, its scope and limitations are directly influenced by what discipline the evidence stems from. In psychology, reliability is assessed through the level of consistency of research findings [2]. Eyewitness testimony places 100% reliance on the human memory, which may be tainted by several psychological influences [3], lowering its reliability. Alternatively, forensic science in law is highly authoritative and relied upon due to its validity and accuracy which has made DNA evidence increasingly unassailable [4], yet problems of reliability still exist due to its circumstantial and subjective nature.

Eyewitness Testimony as Legal EvidenceEdit

Eyewitness testimony in legal terms refers to an account provided by people who have witnessed first-hand the event under trial [5]. In a large share of criminal cases where reliable evidence may be scarce, courts often turn to eyewitness statements to secure their final conviction. Despite it being the main form of evidence in many cases [6], reports indicate that they can be severely inaccurate and are responsible for over 70% of the world's documented false convictions [7], resulting in an inherent trade-off between relevance and reliability.

Stabbing incident witness, Brisbane - 1942

Psychology of Eyewitness TestimonyEdit

The reliability of eyewitness testimony can be skewed by a variety of psychological factors; even everyday bodily influences such as anxiety/stress, memory decay and poor eyesight have been shown to influence false testimony [6]. Through research, psychologists have concluded that eyewitness evidence can be contaminated by an individual’s visual perception and may lead to incorrect reconstructions of the crime [8]. An often heavier influence to false testimony is “eyewitness talk”, whereby witnesses discuss their recollection of events among themselves and subsequently alter their own memory based on the evidence of their fellow witnesses [6], resulting in their inability to differentiate between their own memory and information learned after the incident [7]. An individual's memory reconstruction may also be inherently biased by their specific cultural background and values [9]. These psychological factors depict how heavily succumbed the human mind is to internal and external influences, no matter how confident the eyewitness may be, and the resulting unreliability of eyewitness testimony in legal trials, despite its prevalence in today’s legal system.

DNA as Legal EvidenceEdit

Legal Benefits of DNA ForensicsEdit

DNA Profiling

DNA profiling has been considered the biggest breakthrough in forensic sciences since the discovery of fingerprinting. Since the 1986 Pitchfork case, the use of DNA in the domain of forensic science has seen itself multiply exponentially. Alec Jeffreys, University of Leicester geneticist, used DNA to convict the double murderer and rapist, Colin Pitchfork, which cleared the name of the innocent suspect, Richard Buckland, making it the first legal case solved by DNA [10]. It is important to note that in this case, DNA was not used as evidence but rather helped the authorities pinpoint a suspect. The results observed and the range of use of DNA technology was applauded by many, popularizing its use in forensic labs worldwide[11].

The late 90's saw fast development in the use of DNA, leading to a normalization of its use in court. However, due to the margin for human error and skepticism, this method was not automatically adopted as the standard in court. This aversion to using DNA led to the creation of operations such as the Innocence Project (USA), whose goal was to use DNA testing positively in order to clear the name of those wrongfully accused. To date, it has helped 326 people in the 70% of cases where wrongful conviction was made due to eyewitness misidentification[12]. According to the Innocence Project, tens of thousands of cases in the USA have benefitted from DNA sampling since 1989. DNA has also changed the face of forensic science to a greater extent, as violent crimes, such as rape and murder, are most often committed by multiple time offenders, leading to the creation of DNA databases which can then be used in future investigations[13].

Drawbacks of DNA ForensicsEdit

Mishandling of DNA ForensicsEdit

DNA forensics are highly perceived as an irrefutable and reliable piece of evidence, which can overturn a defendant's adjudication[14]. In fact, any biological evidence collected at crime scenes can be analysed through DNA testing[15]. Although, this practicability has led to faulty forensic analysis of DNA, which have caused many wrongful convictions. In 2013, the New York medical examiner's office reviewed more than 800 rape cases that may have involved crime investigators who were introduced to reports with mishandled DNA evidence[14]. Mishandling of DNA evidence may include the swapping of items within labs, cross-contamination, or the disregard for certain required lab protocols.

“Just because it’s DNA doesn’t mean it’s good science.”[4]

-American biologist and the founder of the Idaho Innocence Project, Greg Hampikian

Misinterpretation of DNAEdit

The nature of DNA forensics, in which a sample can include a mix of many potential suspects, makes it very difficult for analysts to distinguish them. In fact, one person's DNA can be found from a place they have never even visited [16]. This is caused by secondary transfer whereby human skin cells shed and get carried to different places by other people[17]. In 2013, Michael Coble, an American geneticist of the National Institute of Standards and Technology in Gaithersburg, carried out a scenario test which asked 108 laboratories if a particular DNA sample was part of the mix of DNA found on a ski mask from a particular crime scene. 73 laboratories inaccurately concluded that the DNA sample was part of the mix found from the mask [4]. These results clearly indicate how DNA evidence is afflicted by the analysts' discretion. Forensic science is therefore, highly circumstantial, meaning that it is subject to interpretation and by itself, cannot be treated as equivalent to scientific truths[17].

Overall ImplicationsEdit

A research report[18], published by the National Research Council states that the true value of forensics lies in the quality of the biological evidence collected at crime scenes and not necessarily solely on its scientific applicability. Thus, the value of evidence is heavily dependent on expert interpretation, as it does not come from scientific data, but rather from conclusions drawn from several possibilities derived by forensic science. Furthermore, psychological contamination of eyewitness accounts must be considered as it can drastically impact its credibility. It is a necessary and worthwhile task to think about the possible flaws inherent in different kinds of evidence and question professional consensus when convictions must be made in life-altering legal cases. Thus, the onus is on scientific and legal professionals to recognize and interpret the true 'value' of all evidence and adopt a holistic approach to evidence evaluation when concluding a legal case.


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  18. Committee on Identifying the Needs of the Forensic Sciences Community, National Research Council (2009). Strengthening Forensic Science in the United States: A Path Forward. Washington, D.C.: THE NATIONAL ACADEMIES PRESS. Available at: [Accessed 1 Dec. 2018].