Home Experience Services Contact Us Seminars Attorney's Guide to Perception Resources
"Is It A Gun? Or Is It A Wallet?"
Perceptual Factors In Police Shootings of Unarmed Suspects

Marc Green

Seminar Available on this topic.

Police officers sometimes shoot unarmed suspects in error. The majority, about 71%1, occur at night under poor visibility conditions. In one common scenario, a suspect suddenly produces an object in his hand. The officer, believing the object to be a weapon, fires at the suspect. The officer then approaches, only to see that "gun" was actually something as innocuous as a driver's license or wallet.

The subsequent inquiry must judge whether the officer acted reasonably. One component of reasonableness is the officer's perception; should s/he have correctly recognized that the object was not a gun. The visibility conditions, especially light levels, usually become a central issue. Investigators often attempt to assess visibility from witness statements such as "It was bright," "I could see three blocks down the street," "There was enough light to read my notebook," etc.

In reality, these statements reveal little about visibility conditions at the time of the shooting. A realistic assessment requires an in-depth analysis of specific situational factors that determined perception. Roughly speaking, these factors can be divided into two categories, sensory and cognitive. Sensory processing is the initial stage, where light enters the eye and forms an image. When people speak of visibility conditions, they are usually talking about light levels and other factors that affect registration of the image information.

However, seeing involves much more than an image, and visibility is not the only factor affecting the officer's ability to discriminate between a gun and an innocuous object. The quality of the image and its contents provide only raw sensory information that the viewer must next interpret. This requires use of memory and of other innate cognition processes, most importantly expectation. Whether the officer will see a wallet or a gun depends as much on his knowledge and experiences as it does on the sensory factors.

Sensory Factors

Inquiries often focus on light levels. They presume that if lighting were sufficient, then the officer should have correctly identified the object. The mistake was then due to negligence or at least improper care.

This logic is flawed because amount of light is only one sensory factor in determining visibility, and it is often a relatively minor one. All things being equal, it is true that people see better in brighter light. But things are seldom equal because many situational factors are more important than overall light levels.

1. People see contrast, not light. In a real sense, people do not see light; instead, they see contrast, the difference in light coming from an object and its background. As the Illuminating Engineering Handbook says, "Contrast detection is the basic task from which all other visual behaviors are derived." Contrast is the property that allows people to detect shapes and contours and ultimately to recognize objects.

Viewers see most objects in reflected light. A source, such as a lamp, emits light that bounces off a surface and reaches the eye. To see contrast well, the there must usually be a large difference in reflectance, the amount of light that is reflected, off the object and background. A piece of coal is highly visible against a white sheet because the coal reflects very little light while the sheet reflects almost all of the light. Further, the coal against a sheet will be highly visible even in dim light. It is difficult to see in fog even in daylight because the contrast is low. Contrast, not overall light level, is frequently the factor that matters more.

Witness statements about general lighting conditions are often irrelevant. The only important light is the light that reflects off the object and its background because that is what determines contrast. A witness statement saying that he could read his newspaper or see people walking a block away reveal little about visibility of the object in the suspect's hand. Moreover, both object reflectance and background change with viewpoint, so a witness standing at a different location may not be seeing what the officer saw.

2. The location of the light is often more important than the amount of light. Light location determines whether the contrast will be negative or positive. Negative contrast (also called "silhouette contrast") occurs when there is a dark object on a bright background. Newspapers, for example, are printed in negative contrast. Positive contrast (also called "reverse silhouette contrast") occurs when there is a bright object on a dark background.

The difference between the light location, and the two types of contrast is often critical in determining object recognition. Suppose the suspect is standing in front of a lighted doorway, car headlight or a streetlight. Then the light is behind the suspect, and the officer will see the object in negative contrast. The object in the hand will appear only as a silhouette and its detail will be invisible. Most importantly, objects of similar general shape will be highly confusable. For example, a wallet and a 25 mm automatic pistol have a similar square outline. An officer who sees a suspect's hand emerge from a pocket with an object having a square profile will have difficulty determining whether it is a gun or a wallet. The straight lines and the square edge are the most salient perceptions.

If the light is primarily in front of the suspect, it will strike the object and reflect to the officer's eye. S/he will have a much better chance of seeing the fine details and discriminating one object from another, i. e., a gun from a wallet. The more reflective the object, the better the recognition. If the object in the hand is dark, however, then positive contrast will be minimal regardless of light position.

3. Lighting affects perception of outline shape and detail differently. As light level decreases, visibility decreases much more rapidly for fine detail than for general shape. It can still be possible to clearly discern an outline but difficult to see the specific pieces or components. For an officer who views the object in the suspect's hand, there might well be enough contrast to see the rectangular shape of a gun/wallet, but there may not be enough contrast to see the fine detail that discriminates a gun from a wallet.

4. Perception of general shape is faster. People mentally process the general shape of an object faster than the fine details. Everyone has had the experience of "recognizing" a familiar person and then starting to wave or say hello, but then realizing that it was actually a stranger with the same general build, hair, etc. This occurs because the visual system transmits information about larger image areas faster from the eye to the brain. Roughly speaking, viewers make initial recognition after seeing the general outline and before the details can catch up.

An officer who must make a potentially life-and-death decision reacts quickly. He is likely to make a decision based on the visual information that is first perceived, which will always be the general outline and shape. If the light is in front of the suspect then the officer will still see the general shape first may be forced to make a rapid decision before the objects details become visible. If the light is behind the suspect, of course, then the general shape is the only information available to decide whether it is a gun or is a wallet.

5. Motion impairs perception. If an object is moving as you click the shutter on a camera, the resulting image may blur. If the suspect is moving his hand, then the object he is holding will produce a similar effect in the eye. The image is degraded, lowering visibility and impairing recognition. Movement most degrades fine details, the very information needed to distinguish a gun from a wallet.

6. Light adaptation impairs perception. The eye sees contrast best when it is adapted to the same light level as the current scene. For example, a person walking into a movie theater during the day will at first see little more than blackness. After a few moments, the theater becomes brighter and objects become visible. This occurs because the eye adjusts sensitivity to the prevailing light level through the process of light adaptation. Visibility loss is greater when the difference between adapting level and viewing level is high and when there has been little time to adapt.

An officer's contrast perception will be reduced if s/he has spent time looking at a brighter area prior to the time when the object must be recognized. Even time spent viewing a streetlight or writing on a sheet of white paper (which is highly reflective) could raise adaptation level and impair visibility of subsequently viewed objects.

While these six factors are important in most situations, there are others that may be important in some specific conditions. One is glare from a light source that is far brighter than the ambient level. As any driver knows, for example, a passing car's headlights can lower contrast perception and may also destroy dark adaptation and lower visibility for several minutes. Another is "masking," which occurs when a patterned background lies behind the object. The viewer will have more difficulty seeing the object clearly amid the clutter. Finally, there are a wide variety of other possible factors, weather, age, viewer eyesight and spectacles, etc., that may also be relevant in some situations.

Cognitive Factors

Perception is the processing of applying stored knowledge to the sensory input and forming an interpretation. Once the image is sensed, the officer must decide whether it represents a gun or some other object based on what he knows and what he has learned through experience. When the sensory information is uncertain or ambiguous, as with low visibility, the cognitive factors grow in importance.

This is a problem-solving task that humans address by using innate cognitive strategies. In psychology they are called "heuristics," general rules of thumb, or "biases." The term "bias" is not meant in the negative sense of everyday language. In cognitive psychology, a "bias" is a reasoning strategy that aids in quickly arriving at a correct answer. In critical situations, it is dangerous to engage in the slow process of conscious reasoning. Instead, the human mind has evolved heuristics and biases to speed the interpretation and to make sense of a complex situation.

Here are some of the major cognitive heuristics and biases. It is important to remember that these are innate and "normal," in the sense that virtually all humans rely on them heavily. Further, they operate unconsciously, so that most people are unaware that their interpretations are affected by such operations.

1. Experience creates expectation. People become skilled at their jobs through learning and development of expectation. Beginners act in a "controlled" mode, where they must stop and consciously think about every decision. Their performance is slow and inefficient and usually errorful. With learning, people switch to a fast and "automatic" mode where responses seem to occur without conscious thought or decision. Part of the shift occurs because the skilled person learns what to expect, and can act without the need for conscious decision-making, such as recognizing objects.

While it is often said, "see is believing," it is equally true that "believing is seeing." Officers who have previously been in situations where a suspect was armed, for example, have more reason to believe that future suspects will be armed and are far more likely to expect to see a gun. However, the experience need not be firsthand. If the station house talk dwells in instances of suspects with weapons, officers shot by suspects, etc., this can also sway expectation in favor of interpreting indistinct objects as guns.

2. The "availability heuristic" primes perception. People must use information stored in memory to make decisions. In a critical situation, they will most likely use the first information that comes to mind. One factor that causes quick recall is that the information was recently acquired. In general, the most recently learned information is what most readily comes to mind, i. e. is most available in memory. Recently read or heard information is the most available and is likely to have the strongest influence on decision-making and on interpretation of low visibility objects.

For an officer going out on patrol, the most available information may originate in the pre-patrol briefing. For example, the officers may be reminded to keep an eye out for a dangerous suspect of a specific description or that there have been reports of violent crimes in a specific area. When the officer later must make a split-second decision, s/he draws on memory in order interpret the ambiguous visual information. S/he is especially likely to perceive a gun if the suspect fits the general description of the dangerous suspect or is located in the area where violent crimes occur.

3. "Confirmation Bias" strengthens preconceived beliefs. Confirmation bias refers to the tendency to seek information that supports an already held belief and to avoid or dismiss evidence that contradicts it. It is perhaps the most powerful cognitive bias, and has been found to be a source of error and accident across a wide spectrum of human activities.

If an officer believes that a person is likely to be a suspect (he matches a description, is located in a high crime area, acting strangely, etc.) s/he will likely form an opinion that the person is potentially dangerous. Once this belief takes hold, the officer will look for evidence to support the belief and ignore, avoid, or rationalize evidence that contradicts it. When the person appears with an object in his hand, it will be interpreted as a gun. This conclusion confirms the initial belief that the suspect is dangerous.

4. Payoffs affect decision-making. A person who must decide under uncertain conditions, such as low visibility, faces the quandary that is depicted schematically in the Table I. S/he must decide whether a "signal" is present or whether there is only "noise." (The terms "signal" and "noise" arose from the "Signal Detection Theory," a mathematical analysis originally used as a model of radar operators - how did they decide whether there was an enemy plane on the screen or merely background noise.) For an officer who sees the object in the viewer's hand, the "signal" is a gun and the noise is an innocuous object.

The situation can produce one of four possible outcomes. The officer viewer will be correct if he says "yes" when the signal is there and it really is ("hit") or says "no" and it really isn't ("correct rejection.") There are also two kinds of errors. The officer can say "yes" when there is no gun ("false alarm") or "no" when there is a gun ("miss.")

The officer must consider the payoff consequences of each response. If s/he makes a correct "yes response" he may save his own life while a correct "no" leaves the situation as status quo. Of course, the officer will attempt to be correct, but, since the situation is uncertain, s/he must also consider the consequences of an error. There are two types of error possible. If s/he makes a false alarm (says gun when there is no gun), the officer shoots an unarmed and possibly innocent suspect and may face legal problems, etc. If s/he makes a miss (does not shoot when there is a gun), s/he, a fellow officer or bystander may end up wounded or even dead.

The officers have no objective way to assign objective values to the outcomes of the "yes" and "no" responses. Is it worse to shoot an unarmed suspect than to have a fellow officer shot? Is there any worse outcome than being shot and killed? If expectation, availability and confirmation effects are operating, then the sum of the mental processing is likely to weigh toward seeing the object as a gun.


Hindsight, as they say, has 20-20 vision. After the outcome of an event is already known, it is difficult to put oneself in the place of the people who must decide on action before the event when there is uncertainty due to poor visibility. The officer confronting a suspect at night has a difficult decision because he does not have the luxury that subsequent investigators have of knowing with certainty whether or not the object is a gun. Instead, s/he must act based on uncertain visual information. To resolve the uncertainty, the officer will draw on the innate cognitive processes that all humans have evolved to help us make quick decision under stress. These heuristics and biases function automatically because there is no time for conscious decision making. There is only time to react.

The reasonableness of action cannot be assessed by generalities about lighting conditions or post hoc analysis based on what is known now, after the event, rather than what was then, before the event. Proper analysis requires a detailed reconstruction of the visual factors described above and an assessment of the cognitive process operating at the time. The task of reconstructing the past can be difficult and investigators are prone to their own set of cognitive biases, as I describe elsewhere2. Ultimately, the reasonableness of any behavior can only be understood by the physical constraints of the situation and by the likely operation normal human cognition.

When a police shooting occur, inquiries frequently draw on an expert in police procedures, etc. However, a scientific and human factors analysis of the situation may be more revealing, more objective and more informative. When a police officer misperceives an object as a gun, it is a perceptual error, not a police error.


1Aveni, T. (2003). Police Marksmanship Under Fire: Paradox And Promise, Law & Order, August.

2Green, M. (2003). Skewed View: Accident Investigation, Occupational Health & Safety Canada, June.

Other Topics
Personal Injury: Road Accidents
  • Is The Moth-Effect Real?
  • Human Error in Road Accidents
  • Reaction Time
  • Let's Get Real About Perception-Reaction Time
  • Why PRT Is Not Like Gravity
  • Vision in Older Drivers
  • Weather and Accidents: Rain & Fog
  • Accidents At Rail-Highway Crossings
  • Seeing Pedestrians At Night
  • Underride Accidents
  • Rear End Collision: Looming
  • Night Vision
  • Distracted Pedestrians
  • Failure To See
  • Perception-Reaction Time (PRT) Programs
  • Twilight (3.3 lux) As A visibility Criterion
  • Human Error And Fault Tolerance
  • Junk Science Meets Impaired Drivers
  • Personal Injury: Warnings & Product Defects
  • Warnings and Warning Labels
  • Warning Effectiveness Checklist
  • The Psychology of Warnings
  • Drugs, Adverse Effects & Warnings
  • Are Warnings Effective?
  • Human Error Vs. Design Error
  • Product Misuse And "Affordances"
  • Safety Hierarchy: Design Vs. Warning
  • Thinking Like A Human Factors Expert
  • Personal Injury: Other
  • Diving Accidents in Pools
  • Falls Down Steps
  • Medical Error
  • Computer & Medical Error
  • Nursing Error
  • Criminal & Police
  • Errors in Eyewitness Identifications
  • Perceptual Error in Police Shootings
  • Eyewitness Memory Is Unreliable
  • Human Factors In Forensic Evidence
  • Intellectual Property
  • "Any Fool Can See The Trademarks Are Different"
  • Measuring Confusion For Intellectual Property
  • Color in Trademark and Tradedress Disputes
  • Color Functionality: A Case Example
  • Forensic Human Factors
  • Seeing Color
  • Determining Visibility
  • "Inattentional Blindness" & Conspicuity
  • Computer animation has perceptual limitations
  • Photographs vs. Reality
  • The Six Laws Of Attention
  • What is "inattention?"

  • | Home | Experience | Services | Contact Us  | Seminars/CLE | Attorney's Guide  | Resources |

    Copyright © 2013 Marc Green, Phd
    Home Page:
    Contact Us