Challenging the use of Pseudoscience in Excessive Force Cases


By Prof William Harmening

Washington University in St. Louis


Anyone who has been on the Plaintiff’s side of an excessive use-of-force case has probably heard testimony from an opposing expert that included a detailed explanation of something called the reactionary gap. It is a concept that has become deeply embedded in police training modules, and used frequently by those experts who support the police in almost every case, no matter what the circumstances. These pro-police experts use pseudoscientific concepts like the reactionary gap―the Tueller drill[1] is another―to confirm a predetermined conclusion and make any force decision made by an officer appear reasonable.


So what is the reactionary gap? The study comes from a group of criminal justice professors at Texas State University.[2] The study was essentially a variation of the Tueller Drill. Police officers progressed through a series of rooms in response to a “person with a gun” call. In each room was an actor, some holding a gun at their sides and pointed toward the floor, and others holding it to their head in a suicide stance. Twenty-percent of the suspects were told to obey the officer’s commands to drop the gun, while the rest were directed to turn their gun on the officer and shoot. The study’s objective was to determine if the suspects could ACT faster than the officers could REACT.


The study allows pro-police experts to argue that because action is quicker than reaction, the officer is always at a tactical disadvantage when the suspect is armed, near-armed, or only suspected of being armed. Suddenly, the officer who shoots the suspect sitting in his car with a gun in the seat next to him, even without the suspect reaching for it, appears more justified and reasonable. It also provides a reasonableness defense in some cases where an officer shoots a suspect in the back. The study bolsters the idea and necessity of an officer being more proactive in their use of deadly force. In a 2013 case in Chicago[3], a case involving a teenager who was shot in the back and killed by a Chicago police officer as he ran away from the officer holding nothing in his hand but an iPhone box, the expert for the City of Chicago, after first arguing that an iPhone box is easily mistaken for a gun, argued that the shooting was justified because the suspect could turn and fire quicker than the officer could defend himself, even with the officer’s gun raised and on-target. The expert used the Action vs. Reaction study to support his position. In the end, the expert was challenged, and that part of his report was disallowed. The City quickly settled the case.    


So how do you defeat this argument in the courtroom? There are three ways; 1) Challenge the messenger, 2) Challenge the science, and 3) Challenge the conclusions. 




Most pro-police experts I have come across are first and foremost firearms instructors, and no doubt very good ones. But in my experience, very few have any academic background or training in the behavioral sciences. A use-of-force event is above all else a psychological exchange between a police officer and a suspect. It cannot be fully understood without understanding the psychological-perceptual experiences of the people involved. The same can be said of eyewitnesses. If the expert witness offers opinions based on the principals of behavior, and if they have no academic background in psychology, forensic psychology, or the science of perception, then they are NOT an expert, and their testimony should be challenged.


Many times you will see in the expert’s CV a long list of certifications and training, even some that may appear to relate to the principles of human behavior. But these should never be accepted on their face without some level of inquiry. You will find that most are very short police training modules―many of them a single day―delivered by other police officers. Police training is seldom based on academic research. It is based on anecdotal evidence and the police community’s perception of best practices. A classic example is the aforementioned Tueller study, an entirely unscientific study that is now presented in the police training canon as gospel. Police training should never be allowed to go unchallenged as an acceptable substitute for real research-based academic training in the behavioral sciences. Certainly these individuals may have sufficient training and education to act as experts in other areas, but the behavioral sciences is not one of them, and any discussion of the reactionary gap―essentially a discussion of human perception, albeit a flawed one―should not go unchallenged.     




The study itself lacks ecological validity. It is one thing to conduct experimental research in the area of perception and then apply it to the use of force, but it is an entirely different thing to try to research in an experimental setting the actual mechanics of force, as this study does. It is simply impossible to duplicate in an experimental setting the variables involved in a police officer shooting and killing a suspect.


But beyond the validity problem, the study used a flawed methodology. The most obvious problem is that the suspects who were directed to shoot knew beforehand what their actions would be, while the officers did not. This advanced knowledge is a fatal flaw to any conclusions reached about a reactionary gap, conclusions that were sure to favor the suspects, and in the end argue for an expansion of the outer boundaries of the reasonable officer’s decision-making.


Another flaw of the study, and one that is seldom shared by pro-police experts in their analyses, is that the results of the study were either statistically insignificant or inconclusive. The researchers found that on average, the suspects were able to shoot in 0.38 seconds, while the officers were able to shoot in 0.39 seconds, on average. If you look only at the suspects who held the gun to their own heads, their average time was 0.40 seconds, slower than the average time for the officers. Furthermore, the researchers used marking bullets to test for accuracy. They found that the suspects scored a “hit” only about half the time, compared to 90% of the time for the officers. Given that the accuracy rate for officers involved in actual shootings is much lower[4], this alone should cause one to question the ecological validity of the study. Also, the one variable missing, and the one that cannot be experimentally manipulated, is the stress felt in these encounters by both the suspect and the officer.


So when the reactionary gap is referenced and used by a pro-police expert witness, the science itself should be challenged. The study falls short of the scientific method and cannot withstand the scrutiny of the scientific community.




Pro-police experts who attempt to confirm a predetermined conclusion by cherry-picking supportive evidence are oftentimes attempting to squeeze the proverbial square peg into a round hole. The reactionary gap study is one of the tools they use to “grease the peg” a bit. But if the science behind the reactionary gap is problematic, then so too will any conclusions supported by the study be. In the Chicago case described above, the city’s expert concluded that a teenager running away from a police officer at top speed could still turn and shoot with more accuracy than an experienced Chicago police officer with his weapon raised and on target. It was an absurd conclusion, and one based on the expert’s use of the reactionary gap study. But there is absolutely nothing in the study that pertains to a running suspect, especially one running AWAY from the officer. You will find that in many cases, if not most, when pro-police experts invoke the reactionary gap in their analyses, they are using it to justify the indefensible, and to support a state of mind narrative that will support the officer’s actions; a state of mind that is difficult, if not impossible to disprove. These experts should be challenged to defend how the study relates to their conclusions, especially when the study itself reached no significant conclusions. Also, I have found that many times these experts will confuse the unscientific reactionary gap with the very scientific and well-researched perceptual delay. The two are very different. The former involves overt behavior (action and reaction), while the latter involves an automatic function of the brain requiring no overt behavior. Care should be taken to determine if the expert is in fact confusing the two, and then challenge them if they are.


Like the Tueller Drill, the reactionary gap, which has been disseminated through non-peer reviewed police publications, has become a staple of police training. Unfortunately the study’s findings―that in SOME situations, a suspect who already knows they are going to shoot at the officer can beat the officer to the shot by 1/100th of a second, but with nearly half the accuracy―has been replaced with the universally accepted principle that police officers are always at a tactical disadvantage when confronting an armed, near-armed, or suspected armed suspect. The study’s journey from inconsequential to axiom was likely spurred on by the researchers’ conclusory statement:


“Our results show that even well-trained officers, who are operating in nearly ideal circumstances, with their guns aimed at a suspect, cannot reasonably be expected to shoot before the suspect raises his or her gun and fires.”


The dangerous implications of this conclusion, which is not even supported by the study’s own results, are that police officers shoot quicker and at further distances, and perhaps even before a real threat is present; and then pro-police experts use the conclusion to support a reasonable officer argument under Graham v. Connor without ever addressing the study’s findings and many problems. Like the Tueller Drill, it is time for the reactionary gap to be permanently closed and replaced with good and verifiable science.


[1] Tueller, Dennis (March 1983), How close is too close? S.W.A.T. Magazine.

[2] Blair, P. et al. (2011). Reasonableness and reaction time. Police Quarterly, vol. 14, Issue 4, pp. 323 – 343.

[3] Estate of Cedrick Chatman v. City of Chicago, United States District Court, Northern District of Illinois, 13 CV 5697.

[4] Aveni, T. J. (2003). Officer-involved shootings: What we didn’t know has hurt us. The Police Policy Studies Council.