§ 8.05. Human Factors and Evidence Collection

Agencies should develop policies and procedures to minimize the negative effects of human factors that can reduce accuracy in evidence collection.


a. Human factors. The term “human factors” refers generally to the application of psychological and physiological principles to workplace processes and systems. In particular, cognitive biases, i.e., systematic errors in thinking that affect decisions and judgments, can affect the reliability of experts. Whenever people make observations and reach conclusions, they can be affected both by the useful shortcuts and heuristics that make work more efficient, and by those that can be counterproductive and lead to errors. It is particularly important to ward off bias in areas in which decisionmaking criteria are not clear, and in which outside information can influence the decision.

b. Policies and procedures. In police investigations, one concern is that human factors can contribute to a tunnel vision that focuses attention on one theory or suspect, to the exclusion of others. Studies also demonstrate that people have a tendency, termed “confirmation bias,” to place weight on evidence that supports their own pre-existing beliefs. Emotions can impact police work as well, including reactions of anger, punitive impulses, or conscious or implicit racial or gender biases. It is crucially important that agencies seek to minimize any negative impact of such human factors on casework. Policies and procedures can minimize the negative impacts of such human factors. For example, standard procedures can require that evidence, theories, and suspects be carefully documented and investigated. Witness accounts can be tested objectively, through corroboration, but also through science-informed techniques like the lineup procedures discussed in Chapter 10. Investigative work can be reexamined or verified by officers who are independent of an investigation, as a matter of standard practice. Agencies should attend to the manner in which human factors can negatively affect accuracy, and develop policies and practices in response.

Reporters’ Notes

The problem of human factors that can affect police investigations negatively has received growing attention due to scientific research as well as evidence that officers make errors leading to wrongful convictions. Cognitive biases affect all human behavior, and many shortcuts that we rely on are extremely useful and time-saving devices. Officers may face large caseloads that place great demands on their time and that make efficiency a high priority. However, a growing body of evidence has shown how cognitive biases can affect the way officers perform their work in negative ways that damage accuracy. Policies and practices can aim to minimize the negative role that such human factors can play.

For example, officers analyzing evidence from crime scenes can be “vulnerable to cognitive and contextual bias.” Nat’l Research Council of the Nat’l Acads., Strengthening Forensic Science in the United States: A Path Forward 8 (2009). Officers’ prior views about a suspect’s guilt may affect how they evaluate evidence. Steve D. Charman et al., Cognitive Bias in the Legal System: Police Officers Evaluate Ambiguous Evidence in a Belief-Consistent Manner, 6 J. of Applied Research in Memory and Cognition 193 (2017). Officers also can share information that can bias other officers; for example, sharing the results of forensic testing can cause officers to place undue weight on other evidence in a case, such as an eyewitness identification. Saul M. Kassin, Itiel E. Dror & Jeff Kukucka, The forensic confirmation bias: Problems, perspectives, and proposed solutions, 2 J. of Applied Research in Memory and Cognition 42, 43 (2013). Officers’ own biases about a case may also affect witnesses, such as when an officer may unconsciously convey to an eyewitness which person in a lineup is the suspect.

There are techniques available to avoid such biases that agencies should explore and implement in policy and through training. In general, policies and practices can aim to build impartiality into the system, by removing potentially biasing information from the process, or by bringing in independent or outside views to bear on the investigative work. Blinded lineups, discussed in Chapter 10, make use of an officer who does not know which person is the suspect, making it impossible to provide any suggestion, intentionally or not, to an eyewitness. Blind proficiency testing or verification in forensic analysis can provide an additional level of independent review of conclusions, and are discussed in Chapter 9. As developed in the forensic-evidence context in § 9.05, procedures can be instituted through policy to improve the flow of information to officers, including procedures that selectively blind them to irrelevant and potentially biasing information. Itiel E. Dror, A Hierarchy of Expert Performance, 5 J. of Applied Research in Memory and Cognition 121, 127 (2016).

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