Injury causation is a poorly understood topic, and one for which no standard instruction exists in medical school curricula. While many causal determinations are straightforward and easily made as a matter of temporality, some are highly disputed, particularly when there is civil litigation or a criminal prosecution associated with the injury.

In cases of traffic crash-related injury, medical negligence, product liability, and injuries and death related to intentionally inflicted violence, the cause of observed injuries is often disputed. The primary reason for the dispute is the inability to demonstrate causality, which, unlike diagnosis, can only be inferred through probabilistic assessment and comparison. The enigma of causality is that no one can “see” it.

In July of 2016 the 10^th Circuit of the United States Court of Appeals ruled, in Etherton v. Owners Insurance Company, (No. 14-1164, 10^th Circuit), that a previously published 3-step approach to assessing injury causation was a reliable and generally accepted methodology for assessing the cause of traumatic injury in individuals, making the 3-step causal methodology part of U.S. case law.

In chapters 11, 14, and 15 (provided below) of our new textbook on causal investigation in forensic medicine (Forensic Epidemiology: Principles and Practice), we describe various applications of the 3-step methodology in the investigation of causation in crash-related injury and death, alleged medical negligence, and criminal abuse, respectively.

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The elements that comprise the 3-step methodology that were described in Etherton are as follows:

1. Plausibility: This first step addresses whether it is biologically possible for the injury event to have caused the condition (this is also known as general causation). Plausibility is a relatively low hurdle to clear in a causal analysis, and is largely satisfied by the lack of evidence of implausibility (impossibility) of the relationship. An example of an impossible causal relationship is the discovery of leukemia the day after a traffic crash, as it is well established that it is not biologically plausible for trauma to cause leukemia. Plausibility is often established with epidemiologic data or information.

2. Temporality: This second step examines the clinical and other evidence of the timing between the onset of the symptoms of injury and the injury event, and must be satisfied to assess specific causation. First, it must be established that the sequence of the injury and the event is appropriate; the symptoms cannot be identically present prior to the event. Further, the onset of the symptoms of injury cannot be implausibly latent, relative to the injury event.

3. Lack of a more probable alternative explanation: This final step examines the probability of the injury or condition occurring at the same point in time, given what is known about the individual from the review of medical records and other evidence, but in the absence of the injury event (also known as differential diagnosis or etiology). First, evidence of competing injury events must be evaluated, and compared for injury risk. Then, the likelihood of the condition occurring spontaneously must be assessed.

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About the Editors

Michael Freeman is a forensic epidemiologist and consultant in forensic medicine, working in civil, criminal, and academic venues. He has provided expert testimony more than 1,000 times in a wide variety of civil cases, including injury and death litigation, product liability, toxic tort litigation, tobacco litigation, medical negligence, as well as in homicide and other criminal matters. Dr. Freeman holds academic appointments at the CAPHRI School for Public Health and Primary Care at Maastricht University Medical Center, Oregon Health & Science University School of Medicine, and Aarhus University, Department of Forensic Medicine. He serves as an Affiliate Medical Examiner with the Allegheny County Medical Examiner’s office in Pittsburg, PA. Dr. Freeman holds a doctor of medicine degree (Med.Dr., Umeå University), a doctorate in in public health with a major focus in epidemiology (Ph.D., Oregon State University), and an MPH degree (Oregon State University), inter alia.

Maurice Zeegers is the director of the CAPHRI School for Public Health and Primary Care at Maastricht University Medical Center+. He maintains a broad network of partners, develops the institutional strategy, policies and supervises the daily management of the Institute, including the financial control, the multidisciplinary cohesion, the scientific quality control and production mechanisms, and the external review cycles. He is full professor of Complex Genetics and Epidemiology and head of the department of Complex Genetics where he and his team perform their scientific studies, within CAPHRI but also within the NUTRIM School of Nutrition and Translational Research in Metabolism. His main interest is in Epidemiology, Analytics, Forensic Epidemiology, Research Integrity, Nutrition, Genetics and Meta-analyses.