Likelihood ratios in psychological expert opinion, and their reception by professional judges
| Author | Eric Rassin,Nurul Arbiyah,Irena Boskovic,Henry Otgaar,Harald Merckelbach |
| DOI | http://doi.org/10.1177/13657127221119545 |
| Published date | 01 October 2022 |
| Date | 01 October 2022 |
| Subject Matter | Articles |
Likelihood ratios in psychological
expert opinion, and their reception
by professional judges
Eric Rassin
ESSB, Erasmus Universiteit Rotterdam, Burgemeester Oudlaan 50, Rotterdam, Netherlands
Nurul Arbiyah
Clinical Psychological Science, Maastricht University, Universiteitssingel 40, Maastricht,
Netherlands
Irena Boskovic
Erasmus Universiteit Rotterdam, Rotterdam, Netherlands
Henry Otgaar
KU Leuven, Leuven, Belgium
Harald Merckelbach
Maastricht University, Maastricht, Netherlands
Abstract
In various countries, forensic scientists have begun to express their expert opinion in terms of
the likelihood of observing the evidence under the primary and under an alternative hypothesis
(i.e. the likelihood-ratio approach). This development is often confined to technical domains
such as fingerprint analyses. In forensic psychological expertise, likelihood ratios are largely
absent. In this contribution, we explain how forensic psychologists can employ likelihood
ratios, and we describe two illustrating cases. We also present two studies in which we exam-
ined how (Dutch) professional judges appreciate psychological expertise framed in likelihood
ratios. Findings suggest that judges (N=39) appreciate a fictitious expert witness report
framed in likelihood-ratios similarly to an opinion framed one-dimensionally. Judges’(N=79)
understanding of a psychological expert opinion framed in likelihood ratios was satisfactory
as measured by self-report and an actual test We conclude that, as is custom in forensic tech-
nical domains, psychological expert opinion can be expressed in likelihoods. Two of the
hypothesised flipsides, namely, lawyers’dislike of likelihoods, and their lack of proper under-
standing, may be surmountable.
Corresponding author:
Eric Rassin, ESSB, Erasmus Universiteit Rotterdam, Burgemeester Oudlaan 50, Rotterdam, Netherlands.
Email: rassin@essb.eur.nl
Article
The International Journal of
Evidence & Proof
2022, Vol. 26(4) 325–341
© The Author(s) 2022
Article reuse guidelines:
sagepub.com/journals-permissions
DOI: 10.1177/13657127221119545
journals.sagepub.com/home/epj
Keywords
Analysis of competing hypotheses (ACH), confession evidence, expert witness, eyewitness
identification evidence, likelihood ratio
Courts may call upon psychologists to give an opinion on the validity, reliability, or trustworthiness of
various pieces of evidence such as eyewitness identification evidence, witness statements, or disputed
confessions. A crucial question that follows is how the psychological expert opinion should be
framed. Consider a case in which two police officers drive a car at night, observe another car speeding,
and try to catch up. Once the police have caught up, they signal the speeding car (now behind them) with
their taillights to pull over. The car stops, and so does the police car. However, the police hit the brakes
gently, and consequently there is now a distance of some hundred metres between them and the speeder.
Therefore, the police drive in reverse, and it takes approximately a minute for the police to approach the
people in the speeding car. It turns out to be an elderly married couple: The woman sits behind the wheel,
and her husband is beside her in the passenger seat. He has had alcohol, and she has not. The police
suspect that the couple swapped places in the short period needed to drive back to them. By doing so,
the husband would evade penalties for drinking and driving. In fact, the police officer in the passenger
seat testifies that he saw a man in the driver’s seat when they passed the car. Eventually, the husband
is charged with drunken driving, and the wife is charged with perjury. A psychologist is asked to give
an opinion on the validity of the identification of the husband as the driver by the police officer.
Which arguments might the psychologist bring forward? (S)he might argue that the police officer
expressed high confidence in his identification, and police officers generally may well be good observers
(see Wells et al., 2020). Hence, these are reasons to assume that the identification is valid. Alternatively,
one might argue that it was dark, both cars were driving at 100 m/h, the exposure time was brief, and the
car had large headrests, so there is reason to argue that the identification is weak evidence. The psych-
ologist might also discuss both pros and cons, and conclude that there is no scientific way of knowing
how strong the identification is. Regardless of which direction the expert takes, this approach will
always be one-dimensional, meaning that the expert weighs the arguments under one single hypothesis
(how convincing is the identification of the male driver?). One problem with this approach is that the
expert basically does the same as the judge (or, in adversarial systems, the jury). That is, they all
wonder how strongly the identification supports the primary hypothesis that it was, in fact, the
husband who drove the car. If the expert reports that the identification is (likely) valid, (s)he implicitly
states that (s)he believes that the primary hypothesis is true. Should (s)he report that the identification
is likely invalid, (s)he implicitly states that the alternative hypothesis (the wife drove the car) is true.
Hence, this approach may lead to role confusion, in that the expert unintendedly takes the seat of the
judge, and indirectly answers the question of whether the suspect is guilty.
Interestingly, more than thirty years ago, in 1988, Wagenaar already argued that this role confusion
can and should be prevented if psychological experts take a likelihood-ratio approach. According to
him, judges need to decide whether the primary hypothesis is true (probability of the primary hypothesis,
given all the evidence; P (H | E)), whereas experts should limit themselves to an opinion on the likelihood
of the evidence, given the hypotheses (P (E | H)). Applied to the identification case, this means that the
expert has to consider two types of likelihoods. First, how likely is it that the police identifies a male
driver when the driver was, indeed, male? Just for the sake of simplicity, one could settle that likelihood
at 100%. Second, how likely is it that the police would identify a male driver when in fact the driver was
female? Estimating this false positive error requires empirical study. When consulted as an expert witness
in a case similar to the example presented above, Wagenaar carried out a study and subjected 210 parti-
cipants to a simulated exposure to the female suspect (i.e. under suboptimal, darkened circumstances). Of
these, 116 individuals (55%) reported having seen a man. Hence, Wagenaar reported that the maximum
326 The International Journal of Evidence & Proof 26(4)
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