Force Science Institute Ltd.





A Survey of the Research on Human Factors Related to Lethal Force

Encounters: Implications for LawEnforcement

Training, Tactics, and Testimony



Audrey Honig, PhD, Psychological Services, Los Angeles Sheriff’s Department

William J. Lewinski, PhD, Professor, Law Enforcement Program, Minnesota State University, Mankato

To effectively train and fairly evaluate the performance of an officer in a tactical environment, we must first fully understand how the brain perceives and processes information. This article will begin with an exploration of how the brain/mind processes routine information, followed by a discussion of the research on the effects of stress on perception. The brain refers to the actual organ contained in the skull that coordinates sensation and intellect, while the mind refers to consciousness/thought or intellect/memory. For our purposes, however, the terms will be used interchangeably. Common perceptual distortions and mistakes of fact will be identified, and their effect on reaction time will be discussed, taking into consideration the scientific and practical limitations governing human performance. Training recommendations designed to reduce both the rate and range of perceptual and processing errors while decreasing response lag time, or the time it takes to initiate a response, will also be proposed. Finally, improved methods for mining memory will be offered with the goal of increasing the accuracy of incident recall. The information will be presented as objectively as possible. It will be up to the reader to weigh the research, including potential organizational and/or political ramifications, and the pros and cons of any proposed changes to policies or practices.


To truly understand and explain human behavior, we must first make sense of the workings of the human mind. How information is perceived and processed ultimately determines both the level of performance and the subsequent memory of the event. The mind processes information from both internal sources (i.e., thoughts and feelings) as well as external sources (i.e., the senses such as visual and auditory). It is important for the reader to know that it doesn’t matter where the information comes from; it is neither perceived nor processed in a vacuum. This is because both perception and memory are active processes. For example, each of us has a set of schemas and expectations that both color and form our view of our world and have significant potential to distort our perception and then recollection of a critical incident. The influence of this is so strong it literally means perception is reality. It is also important to remember that traumatic incidents, by their very nature, will result in some degree of perceptual distortion and memory impairment as an accompanying feature—usually, the greater the stress the greater their occurrence (Grossman & Siddle, 2005). This means that any two witnesses viewing the exact same incident can, and often do, have widely different perceptions of the event (Loftus, 1979). Their subsequent accounts of and responses to the incident are also then likely to vary. The same is true of law enforcement personnel involved in a tactical situation. Heightened levels of stress, combined with increased elements of both focused attention and distraction, further magnify this effect (Morgan, 2004).


Characteristically, two types of errors exist that are related to perception and that subsequently effect both performance and memory. Type I errors, or false negatives, result from rejecting something that should have been accepted. An example would be failing to identify a suspect who does, in fact, have a firearm, resulting in actions or lack of actions on the part of the officer that may lead to that officer subsequently being shot or at least missing important clues. Research suggests the typical, false negative rate for officers is approximately 4% in a high-stress and rapidly unfolding situation such as a shooting (Lewinski & Hudson, 2003). A Type II error, or false positive, occurs, for example, when an officer incorrectly perceives that a suspect has a gun and, hence, responds with deadly force only to find that no gun exists. The false positive rate for such incidents averages 9% based in laboratory research (Lewinski & Hudson, 2003). Recent research just completed by Aveni et al. and still being analyzed indicates that in simulation testing, Type II errors, depending upon the department and the training and experience of the officer, may be as high as 30 to 40+%. Both types of error are inevitable and are inversely related; as the probability of one goes up, the probability of the other comes down. Subsequently, efforts to mitigate one type of error necessarily result in an increase in the probability of the other type of error.


Lessons Learned


• Perceptions and recollections are colored by prior expectations.


• The greater the trauma, generally, the greater the risk of perceptual distortion and memory impairment.


• Two types of error are inevitable and inversely related: reducing false negatives (Type 1 error) (e.g., failing to identify a weapon when one exists) automatically results in an increase in false positives (Type 2 error) (e.g., seeing a weapon when none exists).


The Science of the Mind


The three critical components of memory are often identified as the three “Rs”: receive, retain, and recall. Information must first be received to be remembered. Attention is the primary process that undergirds and determines what we will receive, retain, and recall. The brain does not have an infinite capacity to observe, and so it normally picks and chooses what to attend to and then simply ignores the rest. Can the reader imagine how quickly they would experience a system overload if they were to attend to each and every blade of grass, leaf, or insect in their immediate environment? This process of selection is generally survival based, having been formed in earlier times when our ancestors’ ability to observe critical aspects of their environment truly meant the difference between life and death. But, like everything else, the system is not perfect. Generally, information cannot be simultaneously attended to and processed from two different sources much less from competing senses (Lewinski & Hudson, 2003; Strayer, Drews & Johnston, 2003; Yantis, 2004). Increased focus on a visual cue, for example, automatically reduces the ability to attend to either a competing visual cue or auditory stimuli. At any given moment, we can see either the forest or the trees but not both at the same time. This perceptual phenomenon is also related to and referred to as figure-ground perception. Readers familiar with Introductory Psychology texts will identify the “Vase/Two Faces” figure as an illustration of this phenomenon. It is difficult to see both the vase and the two faces at the same time. To identify both percepts requires the viewer to shift back and forth between the vase and the two faces.




In law enforcement, officers are still human and cannot perceive two elements of equally high significance at the same time. However, an officer’s training and experience will provide for greater visual or auditory attention and acuity. The experienced officer will pay varying levels of attention to the elements in the encounter, depending on the situation at the moment and the officer’s assessment of the relative importance of each cue to their basic survival (Hsieh, 2002; Yantis, 2004). This characteristic of the brain’s functioning has often been referred to as tunneling. The more technical name for this phenomenon is selective attention. One consequence of this tunneling is that while the brain is attending to a particular internal or environmental cue, it may fail to observe another, theoretically equally important piece of information. The reason for this is that during selective attention, while the person is intently focused on one element in the environment, the perceptual system not only ignores other elements but also actively works to suppress their interference in distracting the person from what they were primarily focused on. Selective attention explains how an officer can fail to see or hear something occurring directly within his or her field of vision or range of sound (Rumar, 1990; Simons, 2003; Strayer et al., 2003; Strayer & Johnston, 2001; Summala, Pasanen, Räsännen, & Sievänen, 1996). In everyday circumstances, it also explains how someone “listening” to the radio, while otherwise engaged in thought while driving to work, can find themselves suddenly unaware of the content of the broadcast that they allegedly just “heard” (Brown, Tickner & Simmonds, 1969; McCarley et al., 2001).


If this selective attention can occur under even the most mundane low stress conditions, imagine its effect under high-stress conditions in which the reader’s life or the life of someone else is “on the line”! Research has found that the impact of intense stress before, during, and after an event affects what an officer remembers and how he or she remembers it (Gold & Greenbough, 2001; Grossman & Siddle, 2004; Lewinski, 2002; Morgan, 2004; Welford, 1980). This then means that information subconsciously deemed unrelated to the perceived threat will have a low rate of recall as attention will have been focused predominantly on the threat and/or on personal survival. Officers, then, because of their focused or selective attention under these circumstances will fixate, or intently focus, on some element of the incident, resulting in a very specific and vivid, though not necessarily accurate, memory for a particular aspect of the incident while limiting their recollection of other facts (Bacon, 1974; Hockey, 1970; Mandler, 1982).