Chapter 10 – Admitting Expert Evidence

VIDEO TRANSCRIPT
“Admitting Expert Evidence”
Chapter 10 of the module guide explores the role and admissibility of expert evidence in
criminal proceedings, and takes a critical look at both the risks associated with this kind of
evidence and how the law addresses them.
In this lecture, we will first introduce expert evidence by exploring its role in fact-finding as well
as the risks associated with its use in proceedings.
We will then examine its admissibility. This will include looking at three necessary requirements,
including helpfulness, reliability, and qualification.
As a general rule, a witness’ opinion is not admissible. It is for the jury, as fact-finder, to draw
their own inferences and value judgments about what has actually occurred. Others’ opinions
would typically be unnecessary, distracting, and an intrusion into the jury’s role.
However, criminal cases can often be complex, and this is increasingly so with the emergence of
various technologies and scientific discoveries that can help determine what has occurred or
establish its significance. In such cases, juries would benefit from hearing from those with the
skills, knowledge, or experience to explain and interpret the significance of various facts.
Because of this, expert evidence is an exception to the rule that opinion is not admissible.
Yet, despite its potential utility, there are in fact concerns with admitting expert evidence.
For one, experts enjoy a high standing, and there is the risk that juries will give their opinion
more weight than it merits, or defer to the expert instead of performing its own, important and
independent fact-finding role.
Moreover, “expert” evidence, despite what its name suggests, is not necessarily sound, and the
potential weight juries will place on it is even more problematic in light of that. Expert evidence
is not always reliable, and might be recognized as such when admitted, or only in retrospect.
Because of this, there have been several high-profile miscarriages of justice—for instance, the
wrongful conviction of Sally Clark—that have occurred as a result of questionable expert
evidence.
Relatedly, while experts are supposed to provide independent opinion, and not advocate for
either side in an adversarial proceeding, this has not always been the case, creating further
concern.
In light of these concerns, the admissibility of expert evidence, and the gatekeeper role that
judges perform, have come under increased scrutiny.
The admissibility of expert evidence is not necessarily straightforward. At its core, it relies on a
positive answer to three questions about experts, which in short can be thought of as:

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Are they needed? Is the body of knowledge about which they speak reliable? Do they have the
expertise?
Each of these will be explored in more depth, with the second question regarding reliability
being the most intricate.
In keeping with the rationale of having experts, expert evidence is only admissible when it
necessary to supplement the jury’s own knowledge, skill, or experience. The key statement in
this respect is found in the case of Turner [1975], which holds that “If on the proven facts a judge
or jury can form their own conclusions without help, then the opinion of an expert is
unnecessary.”
Often, the jury will be deemed competent enough to assess the facts or evidence themselves,
and courts have guarded against expert evidence being used in these areas.
For instance, this is the case in assessing defendants’ mental states when establishing mens rea.
As demonstrated by the case of Henry [2006], evaluation is left to juries’ own experience and
knowledge even where defendants’ characteristics—for instance, their IQ or mental age—
derogate some ways from the norm.
Yet, there does reach a point at which expert evidence is necessary. With respect to mental
states, the courts recognize that expert evidence is appropriate where defendants suffer from
recognized mental disorder, as may be the case in assessing defences of insanity of diminished
responsibility.
Medical and scientific matters are therefore obvious examples of where expert evidence will be
necessary. However, the necessity of expert evidence is not limited to these cases, and there are
no closed categories with respect to future development. Expert evidence may be needed with
respect to a variety of tasks, for instance, questions of foreign law, voice identification, and so on.
While a need for outside expertise is a necessary condition for expert evidence, an assessment of
admissibility must also ask whether the subject matter about which the witness testifies
constitutes a reliable body of knowledge or experience. This is especially important in light of the
aforementioned concerns about this kind of evidence.
The important, and somewhat contentious, question in this respect is what will be considered
reliable, or against which standard is that to be assessed?
The common law remains the authority on this point. As the court in Dallagher [2002] held,
evidence will be admissible in this respect only if it is “sufficiently well-established to pass the
ordinary tests of…reliability.” It is not clear, however, what the ordinary test of reliability means.
Because of this, the common law has been criticized for being ambiguous, inconsistent, and not
demanding enough. Indeed, the court has shown willingness to admit evidence from experts
whose views and techniques were dismissed by the majority of those in the discipline as
unreliable, as was the case in Robb.
In response to this, the Law Commission, in 2011, issued a report recommending the creation of
a statutory test of admissibility. This test would have set out, in statute, a number of broad
criteria with which judges would be required to assess the reliability of expert evidence.
Though not enacted, the Law Commission’s recommendations were incorporated into a Practice
Direction which encourages judges to consider a variety of criteria in conducting their
assessment. The direction lists eight factors in 19A. For example, it asks judges to take into
account “the extent and quality of [the expert’s data], and the validity of the methods by which
[that data] was obtained”. So too does it ask them to consider “whether the opinion takes proper
account of matters, such as the degree of precision or margin of uncertainty.”

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The direction also sets out, in paragraph 19A.6, potential flaws that the court should be mindful
of—for instance, whether it relied on an examination, technique, or method that was not
properly carried out or not appropriate for that case.
It might be wondered, however, how well-placed judges are to perform this kind analysis given
the complexity of the fields in which experts may operate, and also the extent to which judges
will actually do so.
Lastly, even if expertise is needed, and there is a body of knowledge from which expert opinion
will be valuable, it is still of course necessary that the individual offering their opinion be an
expert.
Determining whether an individual constitutes an expert may be straightforward in many cases.
The Court of Appeal in Robb [1991] indicated that, when it comes to established, academic
sciences like medicine, or established, regulated professions like engineering, it is simply a
matter of asking whether they are accepted as a qualified member of those fields.
However, individuals can of course be experts in areas without the same level of formal
designations. Experts may be needed to comment, for instance, on the value of a commodity, or
the artistic merit of particular works. In such cases, the question the court will ask is simply
whether the individual has become sufficiently skilled or knowledgeable through their own
study or experience.
The old case of Silverlock (1894), for instance, involved a solicitor who, outside their professional
role, studied handwriting in their spare time. They were, in that case, accepted as being able to
provide expert evidence on handwriting.
In all then, if these three requirements are met, the court can admit expert evidence.
Nonetheless, recall that it remains for the jury to assess and accept that evidence. In this regard,
judges are generally not required to provide direction to the jury, other than to make clear that
the jury need not accept the expert’s opinion and that it is for them to decide.
This lecture has reviewed the role and risks of expert evidence, and the basic requirements for its
admissibility. After completing your readings, be sure to review the learning objectives set out in
your module guide, and test yourself against the questions available as part of this chapter.

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