On October 2, 2018, police psychologist Laurence Miller took the stand to testify in the defense of Jason Van Dyke, a white police officer who shot Laquan McDonald, a black teenager from Chicago’s West Side, in 2014. The facts of the case didn’t look good for Van Dyke. A 13-year veteran of the Chicago Police Department, he had shot McDonald 16 times within seconds of exiting his squad car. Though McDonald was holding the knife he had reportedly used to damage a patrol vehicle, the 17-year-old was shot while walking away from cops in the middle of the street. None of these events were in dispute. The shooting had been captured by another police vehicle’s dashboard camera, and Van Dyke was charged with first-degree murder, 16 counts of aggravated battery with a firearm, and one count of official misconduct.

The 67-year-old Miller, who lives in Boca Raton, Florida, was not there to argue about external events. Instead, he asked jurors to focus on Van Dyke’s perception of the shooting. With the lights dimmed in the crowded courtroom, Miller presented a slideshow titled, “The Neuropsychology of Deadly Force Encounters.” In life-and-death situations, he explained, the body’s stress response can distort cognition, perception, and memory. For police officers, this can lead to a phenomenon he referred to as “deadly force mindset,” where the officer, flooded with neurotransmitters and hormones that ensure survival of the moment, feels his or her only option is to kill or be killed. Van Dyke, Miller argued, should be found not guilty of the charges against him. Jurors scribbled notes throughout Miller’s testimony.

The use of psychologists in courtrooms is nothing new, of course, and defense attorneys and prosecutors alike have long relied on expert testimony to help divine the complex soup of chemical hormones and electrical impulses that drive human behavior. But the use of psychology in the defense of police officer shootings is less common — in part because so few officers ever end up on trial. Only 96 officers in America have been arrested for murder or manslaughter for an on-duty shooting since 2005, according to Bowling Green State University criminologist Philip Stinson, and such cases rarely make it to a courtroom. But when they do — and some experts suggest it is likely to become more common with the proliferation of cameras documenting police-suspect interactions — testimony on the unique and stressful psychological tableau of police work has become a staple for the defense.

And yet, some psychologists argue that while the biology of stress is well established, its connection to deadly force is far less clear. While officers can experience cognitive and perceptual impairments, like tunnel vision and dissociation, during deadly encounters, researchers ultimately know very little about what role they play in the decision to use deadly force. In the absence of rigorous science, psychologists are dubious of using the neurobiology of stress in defense of police officers who kill.

“The defense used what seems to be an exculpatory argument, though not actual data, to say, ‘You shouldn’t be responsible because this is the level of stress on the job,’” said psychologist Phillip Atiba Goff, a professor at the City University of New York’s John Jay College of Criminal Justice and cofounder and president of the Center for Policing Equity, a think tank that studies racial disparities in criminal justice policy. “This is a bad area for science to be in.”

On October 5, jurors in Chicago ultimately found Van Dyke guilty of 16 counts of aggravated battery with a firearm and second-degree murder. A sentencing date is pending, and while Van Dyke’s attorneys were still angling last week for a new trial (the judge denied those motions), in a phone interview after the original trial, Miller suggested that his testimony may have at least partly prompted the jury to deliver a verdict of second-degree, rather than first-degree murder. That’s a lesser charge, and one that would seem to suggest, legal experts say, that jurors believed that Van Dyke felt himself to be in danger, even if his reaction was unreasonable.

Whether or not Miller was pivotal in prompting that conclusion is impossible to know, but several psychologists and neuroscientists interviewed for this story suggested that if jurors were influenced by Miller, it was based on shaky science at best.

For his part, Miller wholeheartedly disagrees. “The scientific database on stress-related cognitive distortions goes back a century,” Miller said. “The neurobiology and neuropsychology of stress is settled science.”

Dr. Laurence Miller, testifies during the Officer Jason Van Dyke trial for the shooting death of Laquan McDonald on October 2, 2018. Visual: Antonio Perez-Pool/Getty Images

Miller’s testimony hinged largely on the crucible of stress and the threat police officers often face.

“If your car is skidding off the road, correct it; if someone is chasing you, elude them,” Miller testified during the trial — adding that there is a twist for first-responders: “Even though their brains are telling them, ‘Run! Get away, save yourself,’ their job is exactly to do the opposite. They have to run toward the danger,” he testified, suggesting that acting against biological imperatives of survival heightens feelings of danger and increases the perception of a threat.

Miller proceeded to describe in detail the function of various brain structures, as well as interactions and feedback loops that make up what’s called the hypothalamus-pituitary-adrenal (HPA) axis, a feature of the neuroendocrine system that regulates a galaxy of biological processes, such as the secretion of cortisol and norepinephrine. During emergency situations, Miller explained, the amygdala sends a distress signal to the hypothalamus, the brain’s command center of-sorts, which kicks the sympathetic nervous system into high gear. The body’s “fight-or-flight” response is then activated, leaving it momentarily marinated in chemicals that help ensure survival.

Miller’s core arguments do have some obvious merit. Daniel Reisberg, a professor of psychology specializing in perception, cognition, and law at Reed College, for example, noted that police officers who have been in a gunfight naturally often describe it as highly stressful. “Our bodies have a fairly well-tuned mechanism that kicks in when we’re under stress,” Reisberg said.

The problem, some researchers — including Reisberg — say, is when Miller and others among a small group of frequent police defense experts venture further into the unsupported, or at the very least, highly speculative frontiers of psychological and neurological science. Kimberley McClure, a professor of psychology and law at Western Illinois University, for example, pointed out that because stress responses are highly individual, Miller would have to clearly establish those factors that may have activated the HPA-axis during those seconds before Van Dyke got out of his car. And while researchers agree that stress can distort perception, there isn’t much peer-reviewed research connecting these distortions with the decision to fire a lethal weapon.

The training officers receive on the use of lethal force has tended to focus on the quality of an officer’s handling of his or her weapon — the loading, unloading, and the giving of instructions, McClure said. “Less research has focused on cognitive factors,” she added, “such as decision-making and memory under stress.”

Cops, of course, are trained to resist the urge to flee the scene, and experts suggest proper training and experience can give police the ability to suppress some effects of stress during threatening encounters. Research has also found that clear and strict departmental policies on when to use force can help eliminate errors in officer discretion. During the trial, prosecutors argued that Van Dyke’s own statements prior to arriving at the scene, as he described them two years later during the psychological evaluation with Miller, suggested an officer primed for violent interaction. “Why don’t they shoot him if he’s attacking them?” he recalled asking his partner when news of a teenager vandalizing a police car first came in over the radio. “Oh my God, we’re going to have to shoot the guy.”

While it’s difficult for researchers to know what cops are thinking during threatening encounters, the factors that influence their decision to use force are slowly being understood. Since researchers are not at the scene of real-world, deadly encounters, they create high-stress, naturalistic role-play scenarios that let the officer decide whether or not to use force, then analyze their decision-making process afterwards. Research like this is new, and it is finding that the most powerful predictor of police using force isn’t levels of stress, but rather a suspect’s “noncompliant” behavior, such as physical aggression or resisting arrest.

In fact, a 2018 study published in the journal Criminal Justice and Behavior found that many officers do not experience stress symptoms during difficult encounters. The study authors assessed use-of-force decision-making by analyzing 91 “debrief sessions,” which occurred after newly recruited officers participated in two hypothetical scenarios: one involving domestic violence and another involving a drunk suspect. While most recruits in the study reported some form of stress-related impairment, only about 11 percent reported perceptual impairments.

Among those who did experience impairments, however, symptoms affected their ability to perform some fine motor tasks and “to successfully execute force techniques.” For these individuals, the authors say, it’s possible that stress could lead to an incorrect assessment of the situation, resulting in injury to officers or suspects. But experts suggested that far more research along these lines — including studies where scientists at universities team up with police departments — would be needed to understand the extent to which these distortions occur in the real world, and what role they’re playing when officers pull the trigger. “What remains unclear,” the authors of the study noted, “is how officers are making these force decisions.”

An animated video portraying Chicago police Officer Jason Van Dyke and Laquan McDonald was shown during the murder trial of Van Dyke. Miller testified that deadly force cases ultimately boil down to what an officer perceived. Visual: Antonio Perez-Pool/Getty Images

Miller has been a psychologist consulting with police departments for nearly 20 years. In 1988, he earned his Ph.D. from CUNY’s department of psychology, where he specialized in neurocognition. He is licensed to practice psychology in Florida, where he also completed forensic examiner training in 1998. Today, he runs his own private practice, and primarily consults with the West Palm Beach Police Department and the Palm Beach County Sheriff’s Office, where he assesses officers after they’ve been involved in deadly force encounters and other distressing situations.

On the stand, Miller said that he has testified — typically for the defense — as an expert witness in four or five high-profile cases. His fee as an expert witness in cases that require travel is $10,000 per day of testimony (he testified for one day in Van Dyke’s case). Van Dyke’s defense also paid Miller $10,000 for a psychological evaluation on April 1, 2016. Multiple experts interviewed for this story say there are only a handful of psychologists who are called upon to defend use of force cases like Van Dyke’s. Psychological and legal scholars are not convinced that the research is strong enough for the courts.

To determine whether an expert witness’s scientific testimony is based on sound scientific reasoning, judges often use what’s become known as the Daubert Standard — named for a 1993 Supreme Court case in which a family sued a pharmaceutical company for birth defects they believed attributable to a prescribed medication. For scientific testimony to meet the Daubert criteria, it typically must be based on peer-reviewed research that is widely accepted by the scientific community.

McClure and other experts argue that the research used in defense of police shootings too often fails to meet this threshold. “We simply don’t have enough scholarship related to officers’ decisions during lethal force encounters to meet the Daubert criteria for science in the area,” McClure said. Upon reviewing Miller’s testimony, McClure noted that she was struck by how few peer-reviewed citations appeared. “It seems premature for experts to offer testimony in these cases.”

Defending his testimony, Miller said the Daubert Standard was first developed to assess other forensic science techniques, like chemical analysis, and that it does not map neatly onto fields like clinical psychology. “Applied to psychology, it usually refers to the use of psychometric tests and measurements for cognition, personality, brain injury, etc.,” he said by email. “Clinicians and lawyers agree that there is great difficulty applying the standard to clinical examinations.” Since Miller did not use any psychometric tests when he evaluated Van Dyke, he does not believe the Daubert Standard is a useful framework for his testimony.

That testimony included citations of research by police psychologists William Lewinski and Alexis Artwohl, who have suggested that perceptual distortions are commonly reported among officers involved in deadly force encounters — and that these distortions give reasonable grounds for pulling the trigger. Lewinski, who declined to be interviewed for this article, runs the Force Science Institute, a research group that studies, among other things, “how the mind works during rapidly unfolding events.” Like Miller, Lewinski has previously been called on by attorneys defending police officer actions at trial, where he explained how stress and distortions can have profound effects on officers’ perception during deadly force encounters.

A New York Times profile of Lewinski put his role in courts more bluntly: “When police officers shoot people under questionable circumstances, Dr. Lewinski is often there to defend their actions,” the paper noted, adding that he charges $1,000 per hour as an expert witness. Lewinski has been accused by his colleagues of disseminating “junk science” on the witness stand, drawing from studies he has published in police trade magazines and effectively bypassing the peer-review process. Asked by the Justice Department to review Lewinski’s research, Washington State University psychologist Lisa Fournier said the work was “invalid and unreliable,” and she questioned “the ability of Mr. Lewinski to apply relevant and reliable data to answer a question or support an argument.”

Miller acknowledged that “much of Lewinski’s work is on the borderline between case study and empirical research.” Case studies are anecdotes that help tell one officer’s story. They are “a precursor to empirical testing,” said Miller, and therefore should carry some weight. But other experts interviewed for this article maintained that case studies are no substitute for systematic peer-reviewed research that helps us understand what thousands of officers think and do. For his part, Miller explained that he immerses himself in the peer-reviewed literature before taking on any case as an expert, using his decades of experience in the field and clinical judgment as references.

Of course, everyone deserves a vigorous defense. “But the defense must also be rational,” said Fournier, who is an editor at the American Journal of Psychology. That means expert witnesses should have no conflicts of interest, refrain from cherry-picking, and only cite peer-reviewed articles. After reviewing Lewinski’s research for the Justice Department, Fournier said that she’s “frightened” by what passes for science in courtrooms when use of force is being defended. Context, she maintains, is key. Scientists know a lot about the human body’s stress response. But they know much less about how stress affects police officers, who are trained to perform their duties under highly stressful scenarios, and what impact it has on their decision to shoot or not.

Demonstrators celebrated the guilty verdict — though Miller believes he might have helped to persuade jurors to opt for the lesser charge of second-degree murder. Visual: Joshua Lott/Getty Images

Though police departments and psychologists have worked together for decades, the American Psychological Association did not formally recognize police psychology as a specialty until 2013. Broadly defined by the APA, police psychology aims to ensure that law enforcement and other public safety personnel carry out their duties safely, ethically, and lawfully. Ethical concerns naturally arise when two different disciplines come together, and some psychologists are concerned about the way expert witnesses present the science of their field in courts.

A 1994 research report from the National Institute of Justice raised concerns about psychologists addressing excessive force only during stress management training. “Does treating it this way encourage the perception that stress is an excuse for excessive force?” asked Ellen M. Scrivner, the report’s author. Goff, the president of the Center for Policing Equity, had an answer to that question: “It’s morally egregious to advance a theory of science that argues because of the job police have, and the way that we train them, that stress should now be exculpatory for law enforcement.”

How fine is the line between excuse and explanation? “Stress or anything else is not an excuse for any kind of misconduct,” Miller said. “I agree that one should not reflexively exculpate an officer (or anyone else) just because of any mental state or condition.” On the stand, Miller testified that these deadly force cases ultimately boil down to what an officer perceived. Now that dash cameras and body cameras are worn by more and more police officers, juries can weigh an officer’s word against other evidence. Miller thinks video recordings are “a mixed blessing for law enforcement,” and that in the case of Van Dyke, the footage repeatedly played for the jury didn’t “accurately portray what the responder was perceiving.”

In the courtroom, Van Dyke’s lead defense attorney asked Miller if there was any scientific literature on the psychology of police use of force. “One of the things I want to make clear today,” Miller said in response, “is that just about all of what I’m going to be testifying to in the area of police psychology grows out of a much larger field that goes back almost a century, on the psychology and neuroscience of the brain’s response to emergency situations.”

Still, while it may be true that scientific study of stress has a long pedigree, other experts suggest that extrapolations made from that science to explain — or excuse — certain behaviors are far more complicated. Expert witnesses have the responsibility to help judges, juries, and attorneys understand what knowledge is out there, said McClure, who recently evaluated the application of scientific research in officer-involved shooting policies. But experts should also have “an appreciation for gaps in the information we have,” she said.

“There are a number of gaps, and it’s important to identify those: for one, so researchers can do more research in those areas, and two,” she continued, “so we can better inform the courts about what we know with a greater level of certainty.”

Zachary Siegel is a freelance journalist covering public health and criminal justice. His work has appeared in WIRED, New York Times Magazine, and The Atlantic, among others. He was a 2018 Guggenheim Justice Reporting Fellow at the John Jay College of Criminal Justice at CUNY and currently lives in Chicago.

UPDATE: This piece has been updated to clarify Daniel Reisberg’s position on Miller’s testimony.