Expert Witness Journal Issue 67 June/July 2026 - Flipbook - Page 97
Cognitive Bias and Expert Evidence
by Tom McNeill, Partner at BCL Solicitors LLP.
developed systems and rules which provide some
safeguards – jury trials are one (take note, Mr
Lammy)!
Daniel Kahneman famously said that people are
machines for jumping to conclusions. This is
(usually) helpful when trying to avoid predators,
for example, but less so when forming a view on
complex policy issues.
However, no safeguards can guard e昀昀ectively
against the relentless tide of human fallibility; and
there are parts of the criminal justice system that
are particularly susceptible to risks from cognitive
bias. The use of expert witnesses is one.
On most occasions we reach our conclusion
intuitively and then use the analytical part of our
brain to justify that conclusion. More intelligent
people might be better at explaining their position
but no better at reaching the right conclusion –
unless they have the discipline to follow an evidencebased approach.
Witnesses generally cannot give ‘opinion evidence’
about matters that the jury can reasonably assess
for themselves. Were it otherwise, witnesses could
e昀昀ectively tell jurors what to conclude, undermining
their independence.
Political bias is a good example because it is easily
understood. We all know of people on the other
side of the political spectrum whose opinions
are obviously tribal or misconceived – while our
political opinions are as objective and well-founded
as the laws of physics.
Expert witnesses, however, are permitted to give
opinion evidence if a subject matter lies outside the
experience and understanding of ordinary jurors
and the witness has the necessary expertise to
express a reliable opinion.
There is no magic about political bias, however. If we
are emotionally attached to an opinion (such as that
she is a good person; or that I am a good person)
the brain’s capacity to ignore or distort evidence to
preserve that opinion is almost limitless. The same
is true if we are attached to an opinion that a person
is bad.
This can make expert witnesses especially
compelling. As well as being able to give opinion
evidence often of fundamental importance to a
person’s guilt or innocence, jurors will often not
have the technical expertise to independently
evaluate that evidence (nor do judges, coroners, or
chairs of public inquiries, by the way).
Even where emotion is not engaged, we are subject
to many powerful heuristics that make us partial
and error prone in systematic and predictable ways
i.e. biased. Should we take the trouble to learn these
biases we are still liable to being partial and error
prone, unless we adopt robust methods to guard
against bias – and usually even then. This is the
human condition.
Sally Clark
Sally Clark’s 昀椀rst baby son died of Sudden Infant
Death Syndrome. When her second child died
similarly two years later, the police were called in
and the investigation proceeded on the assumption
that two cases of SIDS in the same family was highly
improbable: murder, they thought, was the only
plausible explanation.
Criminal Justice
The criminal justice system recognises the issue
of bias, in a general sense, and over the years has
EXPERT WITNESS JOURNAL
Experts concluded that both deaths were caused
by deliberate acts, either by shaking or smothering
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