Observations on Expert Witnesses in the Derek Chauvin Trial

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On Thursday 21 April 2021, a 12-member jury in the US District Court of Minnesota found Former Minneapolis Police Office Derek Chauvin guilty of the murder of Mr George Floyd. Mr Floyd died on 25 May 2020 when Mr Chauvin along with three other policemen arrested Mr Floyd on suspicion of using a counterfeit twenty-dollar bill before kneeling on and restraining Mr Floyd for more than nine minutes, during which he became unconscious and died. The case has sparked Black Lives Matter protests internationally, and video footage of Mr Floyd’s death has become representative of fraught race, as well as civilian-police, relations in America and elsewhere.
Mr Chauvin applied a degree of force that exceeded what was reasonably necessary (i.e. failing the “objective reasonableness of fear” standard used to assess the propriety of police conduct) and caused the death of Mr Floyd through asphyxiation. The Prosecution relied heavily on the detailed expert witness analysis of the circumstances and cause of Mr Floyd’s death as well as the accepted standards of police conduct. We review the Prosecution’s use of expert witness evidence and what made the court testimony of these experts so persuasive.  

Medical Evidence

Dr Martin Tobin

Dr Martin Tobin is a Lung and Critical Care Specialist at Edward Hines Jr. VA Hospital in Hines, Illinois. The expert stated that his analysis of the situation aligned directly with his expertise in the forces involved in physiology. Dr Tobin provided evidence that Mr Floyd died from lack of oxygen. The expert relied on an animated 3D reconstruction of the event as well as stills from video footage to simulate the effects of law enforcement’s restraint of Mr Floyd.

Dr Tobin’s evidence was particularly persuasive due to the calm, controlled, and logical manner in which he delivered his testimony. When answering Prosecution’s questions and during cross-examination, the expert spoke directly and plainly about key issues of fact, responding with a clear “yes” or “no” before giving the reasons for his response. The promptness of Dr Tobin’s responses demonstrated his clarity on the matter and assisted the Prosecution to draw out its chain of reasoning with an apparent flow of logic.

In addition, Dr Tobin offered the court an accessible and clear model of technical medical concepts relevant to understanding how Mr Floyd’s breathing had been restricted. Dr Tobin’s evidence on the asphyxiation of Mr Floyd began with a short but concise description of the mechanisms universally required for respiration, outlining a simple two-axis model of breathing reliant on horizontal ‘pump handle’ and vertical ‘bucket handle’ expansions. The expert then applied the circumstances of Mr Floyd’s restraint to this model: the forces on Mr Floyd’s body that day had prevented him from operating the front-to-back ‘pump handle’ expansions of the chest required for the functioning of both left and right lungs.

Watch part of Dr Tobin’s explanation here:

Dr Tobin’s findings also refuted the defence team’s theory that Mr Floyd died of complications related to Fentanyl consumption. The expert testified that Mr Floyd’s breathing rate before falling unconscious was at 22 breaths a minute rather than 10 breaths, the reduced number of breaths symptomatic of the active reduction of breathing under the influence of a fentanyl overdose.

Dr Tobin ‘s responses during cross-examination were firm yet unflustered. He further bolstered his standing as a witness through calmly correcting misinformed ideas of medical phenomena implicit in the questions put to him. When asked whether Mr Floyd could have died of other Fentanyl-related reactions, Dr Tobin did not hesitate to respond that though possible, Fentanyl’s intervention with a person’s respiratory centre in particular could only occur through one neuropathway and therefore with the same set of symptoms for each person.

Watch part of Dr Tobin’s cross-examination here:

Dr Andrew Baker (Medical Examiner)

Dr Andrew Baker has worked as chief medical examiner at Hennepin Count, Minneapolis since 2004.  Dr Baker stated that his autopsy showed Mr Floyd had heart disease and high levels of fentanyl in his blood at the time of his death, but that the subdual, restraint, and neck compression of Mr Floyd was the primary cause of his death.

Chauvin’s defence team put pressure on Dr Baker to delineate clearly the role of drugs and heart disease on Mr Floyd’s death. The expert responded by distinguishing between the “primary mechanism” for the death and its “conditional factors”: in light of the Mr Floyd’s conditional factors (heart disease and fentanyl in the blood) at the time of his death, actions of law enforcement “tipped him over the edge” and constituted the “primary mechanism” of Mr Floyd’s “cardiopulmonary arrest”, terms he said were “fancy medical lingo for the heart and lungs stopped”.

Watch part of Dr Baker’s expert testimony here:

Forensic Pathologist Dr Lindsey Thomas

The Prosecution also called Consultant Forensic Pathologist Dr Lindsey Thomas to provide expert witness testimony. Dr. Thomas worked as a Forensic Pathologist for the Hennepin County Medical Examiner’s Office in Minneapolis before retiring in 2017. Her evidence supported the view that police conduct rather than drugs were the cause of Mr Floyd’s death, in particular that “asphyxia or low oxygen” was the “primary mechanism” of Mr Floyd’s death.

Dr Thomas’s responses were effective not only because of her controlled and direct manner but also as a result of her clarification of the distinction between various medical terms. The expert explained that ‘asphyxia’ is distinct from ‘cardiac pulmonary arrest’ (which refers to a sudden cardiac occurrence) in that asphyxia involves a cessation of the functioning of the lungs and heart as a breathing system. Since asphyxia could not include heart attack or heart arrhythmia, Mr Floyd’s death could not be attributed to cardiac abnormalities that usually result in those symptoms. Her explanation of the mutually exclusive relationship between these terms hence not only clarified her findings but also drew out her conclusion that Mr Floyd’s arrest and restraint were the causes of his death.

Watch part of Dr Thomas’s court testimony here:

Police Conduct

Sergeant Jody Stiger (Use of Force Specialist)

The Prosecution called on Sergeant Stiger’s experience as a reviewer of use of force investigations for the Los Angeles Police Department in their examination of whether the extent of police force had been necessary or reasonable. Sergeant Stiger concluded that, given the lack of resistance from Mr Floyd, and his repeated comments about discomfort and pain, “no force should’ve been used once he was in that position”. The actions of Chauvin, in his view, fell well below the legal standard of “objective reasonableness”.

Sergeant Stiger responded to questions by explaining the connection between his observations of the evidence and his opinions about the conduct of the policemen. One of the most significant of Sergeant Stiger’s opinions was that the use of force on Mr Floyd constituted “deadly force”. When prompted as to why he believed this, the expert first described his perception of the material evidence (“Mr Floyd was in the prone position, he was handcuffed and he was not attempting to evade, he was not attempting to resist”) and how this scenario, in his knowledge, could have led to Mr Floyd’s death (“the pressure that was being caused by the body weight… would cause positional asphyxia, which could cause death”).

Sergeant Stiger was clear to specify the bases of his opinions. When he stated that the dangers of positional asphyxia had been known to the Police Department and included in Police training for at least 20 years, he explicitly referenced a date and source of this belief, namely his own experience of training on the topic in 1994.

Watch part of Sergeant Stiger’s expert evidence here

Police Chief Medaria Arradondo
Arradondo became Chief of the Minneapolis Police Department (‘MPD’) in 2017 after 28 years in MPD first as a Patrol Officer then as head of a team dedicated to the internal review of allegations of officer misconduct.

Like Seargent Stiger, the Police Chief gave a clear explanation of the connection between the evidence he examined and his opinion that the actions of Chauvin did not reflect the policy, training or ethics and values of the MPD. Arradondo stated that Mr Floyd’s apparent lack of resistance as well as his vulnerable physical positioning on the street tarmac in the video were clear signs of Mr Floyd’s submission to the arrest. He therefore considered the use of force unnecessary for the purpose of subduing Mr Floyd and therefore altogether excessive in the circumstances (“it should have stopped”).

To clarify the pacifying purpose of the arrest restraint practice, the expert used as a contrasting example what he perceived to be the reasonable restraint and force used on Mr Floyd prior to the policemen positioning Mr Floyd in the prone position.

Watch part of Police Chief Arradondo’s expert evidence here:

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