The Role of the Expert Witness in Developing “New” Law

by Christopher Bruce

This article was originally published in the spring 1998 issue of the Expert Witness.

One of the most exciting aspects of working in civil litigation is that participants in the legal system have the opportunity to influence the evolution of the law. Although some changes in tort law are imposed by legislatures, most developments are litigation-driven.

This raises a question which I have not seen asked anywhere else: Should arguments about changes in the direction of the common law be left to those trained in the law – judges and lawyers – or is there a role in this process for the testimony of “expert witnesses?”

I raise this issue as a result of my experiences with the evolution of two principles in damage assessment: the calculation of the dependancy rate in fatal accident actions and the calculation of the lost years deduction in serious personal injury claims.

Briefly, the two issues are these: In the case of the calculation of the dependancy rate, it is commonly accepted that the surviving spouse would have benefitted from approximately 70 percent of the (after-tax) incomes of each of the deceased and the survivor – with the remaining 30 percent having benefitted the deceased alone. What is not agreed, however, is whether the 30 percent of the survivor’s income which would previously have benefitted the deceased should now be deducted from the survivor’s loss of dependancy. (When this deduction is made, it is said that a “cross dependancy” approach has been used; whereas when the deduction is not made, it is said that a “sole dependancy” approach has been used.)

In the case of the calculation of the lost years deduction, the argument is that a plaintiff whose life expectancy has been shortened will not need to be compensated for the full value of the income lost during the years which he/she will not now live. Numerous theories have been put forward for the determination of the deduction which should be made – ranging from the deduction of only those components of income absolutely necessary to the maintenance of life to the deduction of the entire value of the plaintiff’s projected expenditure on consumption (i.e. deduction of the entire value of income except savings).

My purpose here is not to argue in favour of one or the other of the approaches to each of these issues. I have done that at length elsewhere*. Rather, my purpose is to ask what the role of economists – and other financial experts, such as accountants and actuaries – should be in the presentation of these issues to the court.

The Role of the Expert: Two Approaches

At least two contrasting approaches to the role of the expert can be defended. The first, which I will call “constructive” (but which others might call “interventionist”), recognises that legal arguments are often informed by developments in other disciplines – notably, philosophy, sociology, accounting, psychology, and economics. Where the arguments being made rely on sophisticated applications of these other disciplines, therefore, there may be a role for experts from those disciplines to testify concerning recent developments in the relevant literatures.

Some proponents of the constructive approach would go so far as to argue that such experts should be allowed to testify concerning what the law “should be.” A more appropriate role, I would argue, is that experts would merely be allowed to explain how the tools of their disciplines could be used to cast light on the issue facing the court.

The second approach, which I will call the “passive” approach, suggests that it is only those with formal training in the law who should be allowed to present arguments concerning potential changes in, or interpretations of, the common law. Hence, the opinions of non-legal experts should not be heard in court. The expert’s only role is to apply the existing law as best as he or she can.

The Constructive Approach

The primary advantage of the constructive approach, as I indicated above, is that theoretical and statistical developments in other disciplines will often be of value to the court in making its decisions. If extensive knowledge of these disciplines is required in order to fully understand the nature of the arguments, it may be preferable to have the presentation made in court by experts.

With respect to the lost years deduction, for example, economists, sociologists, and statisticians have considerable expertise with respect to both the definition and measurement of concepts such as “consumption” and “basic necessities.” And with respect to the measurement of dependancy rates, economists, sociologists, and psychologists have all written extensively about interpersonal relationships between spouses within marriage.

The primary danger associated with the constructive approach is that the expert will be tempted to stray beyond his or her area of expertise and begin to comment on matters requiring legal training. The first step in avoiding this problem is for the lawyer who has retained the expert to recognise that certain types of expert testimony can be construed as legal argument. Much of the testimony of experts in Canada concerning dependancy rates and lost years calculations, for example, has implicitly represented an argument concerning what the law “should be” – not because the expert saw that as his or her role but because the expert (and the retaining lawyers) had not recognised that that was what the expert’s testimony implied.

The Passive Approach

There are two advantages to the passive approach. First, it avoids the problem that the expert will stray outside the boundaries of his or her discipline. Second, if the law is well established, the expert will be able to avoid unnecessary testimony concerning possible alternative scenarios which have previously been ruled to be irrelevant. (For example, no Canadian economist would consider “wasting” the court’s time arguing that a tax gross up should be allowed on a loss of income claim, as the Supreme Court has clearly ruled that such a gross up will not be allowed.)

On the other hand, if the law is still evolving, the passive approach encounters two debilitating problems. First, any attempt to extract a straightforward rule from the decided cases is virtually doomed to failure. This is clear in the cases of both the dependancy and the lost years calculations. In both cases, there have been virtually as many different rulings as there have been judicial decisions. For anyone, lawyer or expert witness, to suggest that they can identify what “the” law is on either issue is presumptuous, if not preposterous. Nor would it be useful simply to adopt a “median” position. In issues like the dependancy rate there is no median position; and in issues like the lost years deduction there is no compelling reason to assume, ex ante, that the median position will prove to be the “correct” one.

Second, as a review of the decided cases on both dependancy rates and lost years deductions will reveal, when litigants first attempt to convince the courts to adopt a new legal principle, they often do not concern themselves with the finer details of those principles.

It is clear in the decided cases with respect to lost years, for example, that litigants and the courts have focussed primarily on the questions of whether such a deduction is required and, if so, whether it is “necessities” or the “costs of living” which should be deducted. Virtually no consideration has been given to the deeper issues of what the terms “necessities” and “costs of living” mean, nor of how one might measure those concepts. In the path-breaking Supreme Court case of Toneguzzo-Norvell v. Burnaby Hospital, the only evidence given by the plaintiff’s expert was as follows:

Q. …But would you agree that your average person … would spend something between 50 to 75 percent of their income on necessities…

A. Surely

No attempt was made to define the word “necessities” for the expert, nor was the expert asked to undertake any statistical research into the issue. Similarly, in another case which is widely quoted, the judge indicated that he had based his decision (concerning the lost years deduction) on the testimony of an expert economist. But when I contacted the economist in question he informed me that his entire testimony on that issue consisted of a brief response to a question put to him in cross-examination – a question to which he had not turned his mind prior to that time.

In the early stages of the development of new legal doctrines, it is common for “loose ends” to be left in this way. It would be inappropriate in my view for subsequent courts to rely too heavily on the “precedents” thereby established. Only when it can be shown that a superior court has turned its mind specifically to an issue, and ruled on it, would it be advisable for lower courts to rely on previously-made decisions in a developing area of law.

Furthermore, until the law has been clearly enunciated, it would seem inadvisable to insist that the expert rely strictly on “precedent” if that expert’s discipline has developed tools which would be of value to the court. Provided the expert testimony is presented as an aid to the court, rather than as an exposition of how the court “should” rule, that testimony may have a legitimate role to play.


It is not uncommon to find areas in the common law in which no clear precedent has yet been established. In some situations, like that of the argument concerning cross versus sole dependancy, this is because very few cases have been taken to court. In others, it is because the issues are so complex that the courts simply have not been able to turn their minds to all of the possible nuances. In these situations, I would argue that it would be irresponsible for an expert to argue that she or he had based a damage assessment on the “decided cases.”

At the same time, the expert must also recognise that his or her role in court is not to identify what the law “should be.” Rather, the expert must restrict her or his role to the presentation of theories or facts drawn from her/his disipline which can be expected to assist the court in making an equitable decision.


*On cross- versus sole-dependency, see Assessment of Personal Injury Damages, 2nd Edition (Butterworths, 1992); “Calculation of the Dependancy Rate in Fatal Accident ActionsExpert Witness, Winter 1996; and “Determination of Personal Consumption Expenditures in Fatal Accident Actions: A Note” Journal of Forensic Economics, 10[3], 1998.

On the lost years deduction, see “Shortened Life Expectancy: The ‘Lost Years’ Calculation“, Expert Witness, Spring 1996; “The ‘Lost Years’ DeductionThe Barrister, December 1996 issue (number 42); and “The ‘Lost Years’ Decuction” Lawyers Weekly, March 28,1997. [back to text of article]


Christopher Bruce is the President of Economica and a Professor of Economics at the University of Calgary. He is also the author of Assessment of Personal Injury Damages (Butterworths, 2004).