AuthorHamirani, Maryam

The capabilities of forensic science have expanded and evolved over the years, facing a number of significant challenges. A main weakness is its susceptibility to cognitive bias. (1) Today, despite remaining a powerful element within the justice system, and playing a key role in establishing and reconstructing events, forensic science much like any scientific domain, faces weaknesses and limitations. These issues can arise throughout an investigation; from when the forensic evidence is first collected at the scene of the crime, until the evidence is presented at court. While recognizing the cognitive and scientific challenges forensic science faces, this paper also demonstrates that the value of forensic science extends beyond the courtroom.


    Forensic science is often assumed to be scientific and impartial. Yet, examining wrongful convictions in the United States reveals that sixty percent of the cases involved flawed forensic testimony. (2) The United States' National Academy of Sciences ("NAS") released a report in 2009, calling into question the scientific validity of many forensic domains, stating that, "[w]ith the exception of nuclear DNA analysis, however, no forensic method has been rigorously shown to have the capacity to consistently, and with a high degree of certainty, demonstrate a connection between evidence and a specific individual or source." (3) Despite this, studies have demonstrated that DNA evidence is hardly incontrovertible.

    In 2016, a report by the President's Council of Advisors on Science and Technology ("PCAST") stated that irrelevant contextual information can bias the interpretation of DNA evidence. (4) In one particular study supporting this, seventeen expert DNA examiners were asked for their interpretation of a DNA mixture previously examined in an adjudicated criminal case. (5) They were only provided with relevant contextual information, but their interpretations were still found to be inconsistent with each other. (6) If mixed DNA interpretation was truly objective, all the expert DNA examiners would have come to the same conclusions. Furthermore, the majority of the examiners that were context-free (unaware of the details of the case), disagreed with the laboratory's pre-trial conclusions. (7) Hence, the irrelevant contextual information surrounding the original DNA evidence may have influenced the interpretation of that evidence, thereby showing a biasing effect of contextual information in DNA mixture interpretation. (8)

    The PCAST report also stated that several forensic techniques should no longer be used in court until sufficient scientific research is done to validate their accuracy and reliability. (9) It was stated that some of the forensic methods used have been assumed rather than established to be foundationally valid based on appropriate empirical evidence. (10) Firearm, fingerprint, bite-mark, and hair analysis are just a few of the methods found to fall short. (11) The assumption that within each of these fields one can rely on the uniqueness of a source has been continuously challenged by studies of variability in proficiency testing and in actual cases. (12)

    Hence, various inquiries into forensic science, empirical research, and actual casework have revealed two major issues with forensic science: first, the lack of an empirical foundation, and second, the susceptibility of forensic decision-making to context and potentially bias. (13) It is important to distinguish the two issues, as these have different roles to play in the forensic science process. This paper examines the whole arc of the process, starting from the crime scene, continuing to the analysis and interpretation of the evidence, and ending with the presentation of the findings. These issues will be discussed to determine how and where they arise in the field of forensic science, and how such problems can impact the fair administration of justice.


    The majority of forensic science still relies on the human examiner to be the instrument of analysis. (14) Therefore, a deeper comprehension of the processes involved in judgment and decision-making, and how they relate to forensic science has to be considered. A practical example of how decision-making processes are vulnerable to cognitive limitations can be seen by examining the lack of interrater reliability across experts and intra-rater reliability over time within the same expert. (15) These terms refer to how consistent conclusions are between forensic experts, as well as within themselves respectively. Many studies on fingerprint analysis have found that examiners' decisions are not always in agreement, (16) and in particular, a study in 2011 found that only sixteen percent of participating experts observed the exact same number of fingerprint minutiae when analyzing the same latent fingerprint mark a few months later. (17)

    The lack of reliability within and between examiners indicates that the identification process can be subjective and that judgments are susceptible to bias from other sources. (18) Often, forensic analysis conclusions are drawn from observations on whether two patterns are "sufficiently similar." (19) These methods often have no quantification, and thus, these judgments lead to subjective observations. Especially problematic are cases containing complex or ambiguous forensic evidence--which crime scene evidence often is. (20)


    The literature in behavioral research has demonstrated that judgments and decisions are subject to numerous influencing variables. (21) The term most often used to describe these in the literature is biases. (22) The two main categories of the biases that will be discussed in this paper are cognitive and motivational. (23) Cognitive biases are the result of unwarranted and illogical inferences. (24) These inferences are a result of mental shortcuts to logical reasoning that individuals develop unconsciously, as a consequence of their experiences. (25) In forensic science, the most common types of cognitive biases that have been studied in the literature are confirmation bias (26) and contextual bias. (27) Motivational biases, on the other hand, include conscious or subconscious judgments, and decisions that are influenced by self-interest, social pressures, or organizational context. (28)

    Research has found that there are seven potential sources by which these cognitive and motivational biases influence the judgments and decisions that forensic experts have to make. (29) A taxonomy developed by Dror illustrates how these sources interact with each other, and can be seen in Figure 1, below. (30) The structure of the taxonomy begins with the foundational level, which relates to basic human nature and the architecture of the brain. (31) The mere fact of human vulnerability to limitations of the brain underpins the subsequent levels of the taxonomy. As we progress up the taxonomy, the sources of bias become more conditional on the specific environmental circumstances and individuals involved. (32) Examples of these are: training (33), motivation (34), organizational culture (35), as well as base-rate expectations. (36) The highest level of the taxonomy presents sources of bias that are specific to the case at hand, such as irrelevant contextual case information, as well as the reference materials and actual evidence from the crime scene and the suspect. (37)

    The concern is that any bias can create flawed decision-making at any stage of an investigation and, consequently, affect the following stages as that investigation progresses. Bias cascade and bias snowball effects are terms that refer to this phenomenon. (39) The bias cascade effect is when bias arises by cascading from one stage to another, e.g., from the initial evidence collection at the crime scene, to the evaluation and interpretation of the evidence at the forensic laboratory. (40) The bias snowball effect increases as a variety of sources are integrated and influence each other. (41) The following section of this paper will discuss each stage of an investigation and what some of the current literature has found in terms of the potential impacts on decision making at each stage of a forensic reconstruction.


    These seven sources of bias have the potential to occur and influence every stage of the investigation, even before the evidence is submitted to the forensic laboratory. At the crime scene, a specialist advisor or a crime scene examiner must initially determine where to look for evidence, and then decide whether what they find is relevant to the case. (42) It then must be determined if sufficient information can be gained from submitting the evidence in to be analyzed, to justify sending it to the laboratory. (43) These decisions often depend on the environment and context at the crime scene; however, the same contextual information has the potential to have an impact upon what is looked for, where it is looked for, and how it is collected. (44) This can be particularly concerning, as these are vital decisions that impact the future 'success' of the investigation, for instance, material that is not collected at the time often cannot be retrieved later.

    The next stage in the investigative process occurs when the evidence arrives at the forensic laboratory. Before it is interpreted and used for identification, it first must be perceived. At this stage, in some environments, experts are potentially surrounded by unnecessary contextual information about the case. This may include knowing the nature and the details of the crime, knowing information about the suspects (e.g., their past criminal convictions), as well as potentially experiencing pressure from other actors within the justice system. (45) In some environments, forensic experts can receive direct communications from these other actors in the...

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