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Legal Mind Case and Commentary No 24: Gestmin and Witness Evidence [Koch HCH, McFadyen K, Crowther-Green H and Milner P, 2019]

20/11/19. Case: Gestmin 5GPS -v- Credit Suisse (UK) Ltd [2013] EWHC 3560

In this commercial case, Mr Justice Leggatt set out a number of issues to be considered in relation to witness credibility. These were summarised in Exall (2016). Leggatt was considering events in 2005/2006 where his assessment of evidence was very dependent on the reliable recollection of witnesses. Exall gave a concise overview of what are called the Gestmin Principles, in a way which is easier to grasp than referring back to the source case.

The Gestmin case referred to a commercial claim that investment advice was negligent, and damages were claimed for loss suffered as a result of making the investment.

Mr Justice Leggatt referred at length to the “obvious difficulty affecting allegations and oral evidence based on recollection of events which occurred several years ago, involving the unreliability of human memory”.

He eventually concluded that Gestmin had failed to make good its case, and dismissed its claim. His comments covered several areas (according to Exall, 2016) illustrated in Figure 1 below: -

Each of these areas are taken in turn below.

1. There is obvious difficulty with oral evidence based on recollections of events which occurred several years ago, and the retrieval of said recollections. This also assumes accurate encoding and sufficient storage.

The court can sometimes place inappropriate reliance on the reliability of memories, and their vividness or claimant confidence and abilities in their recollection. This needs to be taken into account.

2. Memory as a mental record is not fixed, and memories are fluid and change especially during retrieval and especially over time. They also are subject to the influence of other similar memories.

This applies equally to ‘one-off’ memories of a shocking, traumatic event or a series of interconnected events e.g., an abusive relationship.

Recall is affected by belief and thoughts, and inevitable bias that can affect anyone. Stress also influences recall abilities.

3. Memory is vulnerable to alteration when a claimant is presented with new information or suggestions, especially when memory is already weak due to the passage of time. Suggestibility is a crucial contaminating factor when seeking valid, reliable recollections. They are also vulnerable to the language that others use to describe the memory e.g. ‘hit’, ‘collision’ or ‘smash’.

4. Memory is affected by desirability for a particular version of events e.g. trauma, disruption. A claimant is unconsciously predisposed to elaborate their narrative, sometimes this is accurate, sometimes not. The more often a memory is elaborated, the more it is believed, whether it is accurate or not.

The experience of litigation affects the process of making a witness statement, albeit unconsciously, again due to social/legal desirability factors and demand characteristics.

5. The passage of time, available documentation and the input from lawyers all affect ‘retrieval’ of memories and the reliability of details. This can explain, for example, how two experts can obtain different narratives from a claimant interviewed one year apart. See point 3 above.

6. The vividness and apparent authenticity and confidence of delivery of memories are not a reliable measure of their truthfulness. The confident demeanour of a witness is not a proxy for the truth. Nor is the opposite the case.

Reconstructing and retrieval of memories can be affected by unconscious processes which reduce their accuracy and reliability.

7. A judge should be careful of placing reliance on witness recollection and of past conversations. Although an individuals’ recall of verbal vs. visual information can vary.

Factual findings are based on inferences drawn from the documentary evidence and known probable facts.

I am grateful and indebted to the summary of Exall (2016) which I have reproduced in part here, with my own clarifications and additions.


The emphases and weight placed on oral witness evidence does not equate to its length and, especially, should not be based on its confident and assertive delivery (Koch, 2016).

Key reliability factors include (a) the consistency of a witness’s evidence with other known facts and evidence, (b), the internal consistency of the witness’s evidence (i.e. does one part marry up with another part) and, (c) consistency of their statement from one time to another (Exall, 2017).

Judges, Barristers and Psychologists are all very aware, from their respective training and experience, that an individual’s ability to represent their reality, both present and recent past (let alone distant past) is heavily affected by many factors – some of which have been described here and elsewhere (Koch, 2018).

The social desirability of accepting what others say at total 100% face value has to be heavily resisted by the professionals listed above despite the tendency to do just that. It has been suggested that perhaps oral evidence with its many vagaries and unreliabilities should be minimal or omitted. However, this would, I’m sure most would agree, reduce the breadth of evidential experience available in court (Koch, 2019).

The general legal argument is that one can’t allege something without other evidence that supports this, which does not appear to be prejudiced or biased, and is not supported by other unprejudiced witnesses. So although it turns into an argument around memory bias, which is a valid discussion, it’s really an argument around not being able to allege something without evidence that supports this that is not prejudicial.

However, it still seems valid to offer witness testimony where there is supportive evidence or supportive testimony from other witnesses that does not seem prejudicial, which should be weighed on its merits, and how compelling it seems. Although memory can be faulty, and people can hold biases and people do reconstruct memory through recall, that does not mean that some truth is not retained, even if imperfect. There is also good evidence that financial decisions enhance memory recall (Muruyama and Kuhbander, 2011). Eyewitness testimony research suggests memory can be subject to biases, but not that it is wholly false.

However, great care must be taken to apply sensible and logical rigour to how oral testimony is weighted and understood, and put in context of other available evidence.


Professor Hugh Koch, Dr Ken McFadyen, Dr Helen Crowther-Green and Dr Philip Milner.

Professor Koch, Dr Ken McFadyen, Dr Helen Crowther-Green and Dr Philip Milner, Hugh Koch Associates, Cheltenham, Gloucestershire, UK.


Exall G (2016) Witness evidence, reliability and credibility: why everyone should read Gestmin (or failing that, my seminar). www.civillitigationbrief. 31/5/16

Exall G (2017) Lawyers, litigation and memory III: The Gestmin Principles applied. www.civillitigationbrief. 10/1/17

Koch HCH (2016) Legal Mind: Contemporary Issues in Psychological Injury and Law. Expert Witness Publications. Manchester.

Koch HCH (2018) From Therapist’s Chair to Courtroom: Understanding Tort Law Psychology. Expert Witness Publications. Manchester.

Koch HCH (2019) Legal Mind Case and Commentary: Publication Directory 2019. Expert Witness Publications. Manchester.

Muruyama, K. and Kuhbander, C. (2011). Money enhances memory consolidation - But only for boring material. Cognition, 119, 1, 120-124.

Previous commentaries have covered:

Reasoning and Logic in Psychological/Psychiatric Evidence

Maintain robust and impartial opinions during the Joint Statement process

To convince or deceive? The analysis of reliable and realistic evidence


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