Medical expert witnesses are ill-equipped to answer questions of legal causation, and need to be taught.

Physicians and lawyers speak different languages on the topic of Causation1. Litigators wanting a legally meaningful response from a medical expert need to understand those differences in some depth and develop a translation programme. Time spent in patient, detailed pretrial explanation of legal concepts to medical expert witnesses may pay big dividends.

Medical and Legal Causation are so dissimilar that counsel must patiently and explicitly educate medical expert witnesses

Our incredulous physician asks, "The deceased patient was 93 years old, and dying of 7 different degenerative disorders, any one of which might have killed him at any time - and you want to know by how many days or weeks the defendant physician’s inadvertent administration of double the dosage of medication on that occasion would have shortened his life?"

"Yes", replies plaintiff’s counsel, unperturbed, "but not with scientific certainty, simply on balance of probability, and with the best evidence available to you today."


Medical concepts of causation:

1. all possible factors
2. most important influences
3. multifactorial interplay
4. proportional contribution
5. alternative theories
6. rigorous scientific validity
7. not yet established

In the 1960s Edward Lorenz contributed "Butterfly Effect" to the development of Chaos Theory: a butterfly flapping its wings in Saint John, New Brunswick causes a thunderstorm in Regina, Saskatchewan.

This effect provides a graphic analogy for the gulf between medical and legal concepts of Causation. If it negligently or tortiously flaps its wings and advances the inevitable thunderstorm by 3 minutes, the butterfly may be held 100% legally responsible for the damage which occurs during those 3 minutes.

Clinicians are preoccupied with the earliest indicators of stormy conditions, taking evasive action if possible and minimising the resulting damage. Epidemiologists and other medical scientists seek to identify and inter-relate all the possible factors which do or might combine to create atmospheric conditions favourable to the thunderstorm: the flapping of the butterfly’s wings is at most of minimal, passing interest.

Tort law is concerned, of course, solely with the damaging effect of the negligent flapping. Litigators and judges are unconcerned with the identity and interplay of actual and possible influences that resulted in the atmospheric instability.

In medical constructs, the butterfly is so trivial and improbable an influence that it can safely be disregarded.


When a butterfly tortiously flaps its wings in Saint John, New Brunswick and causes a thunderstorm in Regina, Saskatchewan, physicians and lawyers have different frames of reference

The physician may offer that the flight of an unusually large flock of migrating birds is an equally plausible explanation of the early thunderstorm, but the law finds such theorising both confusing and irrelevant. To medical ways of thinking, probable proximate cause is of mild interest at best.

Similarly, inevitability is a trivial detail in the promotion and defence of personal injury actions, but central to the way physicians think about the evolution of pathological processes: if deterioration, disablement or death would have occurred anyway, why is it so important that it was a little sooner rather than later? After all, we presently have no way of curing degenerative diseases.


Legal Causation curriculum for medical expert witnesses:

1. narrow focus
2. any influence, however small
3. precipitate, hasten, aggravate
4. total responsibility
5. "equally plausible" irrelevant
6. >50% probability
7. best available evidence

Grounded in Western medical science, academic physicians in particular are reluctant to leave the comfort of agnosticism to adjudicate 50.1% probability in a unique set of clinical circumstances. Practising the art of medicine means taking management decisions in the face of uncertainty, but this pragmatism is far removed from pronouncing a causal relationship on balance of probability.

Divergence of medical and legal thinking on causation will likely increase rather than lessen. Many physicians are only just getting familiar with the epidemiological "web of causation", in which disease is produced by the interplay of a number of factors, any one of which is neither necessary nor sufficient2. Not all smokers develop squamous-cell carcinoma of the lung, and lifelong non-smokers are not immune. Neverthless, there is a direct relationship between probability and total packs of cigarettes ever smoked.

Ask your medicolegal resource physician to translate the central legal causation issues into medically meaningful questions which the expert witnesses should address

Tort law asks the expert witness to judge whether, in this particular set of circumstances the outcome would have been materially different, absent the event under scrutiny. Neither formal education nor daily clinical practice prepares the medical expert to address such a question.

Copyright © 2008 Electronic Handbook of Legal Medicine