Where blind faith is placed in the relevance, cogency, scientific integrity, and probative value of an expert’s: conclusions; underlying theory; data; and assumptions, there is a risk that the advocate will surrender his judgment, lose his bearings in the case, and that the legal and evidential foundations upon which the case is predicated, will be exposed by his opponent at trial as amounting to nothing more than wishful thinking, rather than blocks of granite.
The advocate should always remember that, ‘It is not the fact of the presence of a mental disorder, or even its severity, that determines testamentary capacity. It is the particular way in which the illness affects a specific testator that decides the issue. These might appear to be statements of the obvious, but sometimes they seem to be ignored… In contentious probate the parties tend to grab at cognitive test results like ship-wrecked passengers from the Titanic scrambling to get into a lifeboat. Lawyers, medical experts, and, dare one say it, judges may also seek refuge there… [However] not all cognitive tests are seaworthy enough for the particular conditions, and some are little better than flotsam…’ (‘Testamentary Capacity’, by Martyn Frost, Stephen Lawson, and Robin Jacoby, paragraphs 13.01 and 14.01).
In The Vegetarian Society & anr v Scott , HHJ Simon Barker QC stated that a key factor in preferring the evidence of the claimant’s expert was that he was ‘familiar with the elements of capacity necessary for a testator to make a will’ whereas the other party’s expert was not. Consequently the evidence of the preferred expert, ‘was the more focused and helpful of the two.’
As Professor Robin Jacoby and Peter Steer remark in their article, ‘How to assess capacity to make a will’  British Medical Journal 335; 155-7, 2 ‘Much litigation could be avoided… if, doctors, when asked by solicitors, assessed testamentary capacity correctly.’
Testamentary Capacity (my talk to ACTAPS) Handout.pdf (carlislam.co.uk)
Paragraph 4.4 [Mental Disorders] of my book the Contentious Probate Handbook, contributed by Dr Hugh Series of Oxford University:
In a testamentary capacity claim, ‘Where the will is rational upon its face…the burden [of proof] shifts to the opposing party to raise a real doubt as to capacity. If that occurs the burden then reverts to the propounder of the will.’ The Vegetarian Society & anr v Scott .
To remove the presumption of validity, the burden is then on those who challenge the will, to show sufficient doubt about the deceased testator’s (‘T’s’) capacity.
The showing of ‘sufficient doubt’ does not require proof that T actually lacked testamentary capacity, merely that the evidence produced shows sufficient grounds for the court to accept there is ‘a real doubt’ as to capacity, Turner v Turner .
In most circumstances, failure by the propounder to produce evidence, results in a finding against the will, Ledger v Wootton  (where the invalidity of the will was decided not on sufficient proof of incapacity but on the defendant’s failure to discharge the burden of proof after real doubt had been raised).
The weight to be attached to expert evidence is entirely a matter for the trial judge, and expert evidence is neither automatically admissible in a testamentary capacity claim, nor necessarily a decisive factor.
The duty of the court is to consider the expert evidence in the light of the facts, not in isolation from them, and where a case involves substantial elements of both opinion and factual evidence the court may accord as much weight to each as it sees fit.
As Lord Justice Mummery stated in Hawes v Burgess  (Court of Appeal), in a testamentary capacity claim ‘… the court has to consider and evaluate the totality of the relevant evidence, from which it may make inferences on the balance of probabilities … I should add a statement of the obvious in order to dispel any notion that some mysterious wisdom is at work in this area of the law: the freedom of testation allowed by English Law means that people can make a valid will, even if they are old or infirm or in receipt of help from those whom they wish to benefit, and even if the terms of the will are hurtful, ungrateful or unfair to those whose legitimate expectations of testamentary benefit are disappointed. The basic legal requirements for validity are that people are mentally capable of understanding what they are doing when they make their will and that what is in the will truly reflects what they freely wish to be done with their estate on their death.’
In Loveday v Renton and Welcome Foundation Ltd  Lord Justice Stuart-Smith stated, ‘In reaching [a] decision a number of processes have to be undertaken. The mere expression of opinion or belief by [an expert] witness, however eminent…[in this case about whether a vaccine could or could not cause brain damage] does not suffice. The court has to evaluate the witness and the soundness of his opinion. Most importantly this involves an examination of the reasons given for his opinions and the extent to which they are supported by the evidence. The judge also has to decide what weight to attach to a witness’s opinion by examining the internal consistency and logic of his evidence; the 6 care with which he has considered the subject and presented his evidence; his precision and accuracy of thought as demonstrated by his answers; how he responds to searching and informed cross-examination and in particular the extent to which a witness faces up to and accepts the logic of a proposition put in cross-examination or is prepared to concede points that are seen to be correct; the extent to which a witness has conceived an opinion and is reluctant to re-examine it in the light of later evidence, or demonstrates a flexibility of mind which may involve changing or modifying opinions previously held; whether or not a witness is biased or lacks independence. There is one further aspect of a witness’s evidence that is often important; that is his demeanour in the witness box. As in most cases where the court is evaluating expert evidence, I have placed less weight on this factor in reaching my assessment. But it is not wholly unimportant; and particularly in those instances where criticisms have been made of a witness, on the grounds of bias or lack of independence, which in my view are not justified, the witness’s demeanour has been a factor that I have taken into account.’
I would add the following observations made by the late Lord Bingham in his article, ‘The Judge as Juror: The Judicial Determination of Factual Issues’ published in his book ‘The Business of Judging’ (which are direct quotations):
- Expert witnesses may be and often are partisan, argumentative, and lacking in objectivity, but they are not dishonest.
- The problem remains: how is a judge faced with conflicting opinions of two or more experts, to choose between them?
- Manner and demeanor give no assistance here, and it is surely that the more truly learned a man is the more ready he is likely to be to admit ignorance and acknowledge inability to provide a perfect solution.
- It is often the superficial expert or charlatan who offers the most confident answer.
- Nor can the choice be based on comparison of the expert’s respective qualifications – frequently the experts’ qualifications are broadly comparable.
- Where they are not, the choice usually lies between one expert whose career has been devoted to the amassing of postgraduate degrees to the virtual exclusion of practical experience in the field, and another with no formal qualifications but a lifetime of experience in handling the commodity or operation in question.
- There is in truth no easy way out, no short cut.
- The only safe way in which a judge can choose between the opinions of experts is on the basis of what they have submitted and in the course of forensic questioning.
- This is as it should be, but it does I think raise a problem. For a judge to prefer the opinion of one expert to another he must understand what they have both said and form a reasoned basis for his preference.
- Usually this gives rise to no problem.
- The conflict of expert opinion may relate to an issue which is not particularly complex, or it may arise in a field of which the judge has previous experience or which he has studied at a level which at least enables him to understand the concepts to which the experts refer and the language they use. But this is by no means always so. The more advanced and experimental a technology the more risk there is of mishap.
- There are in my view times when the ability of judges to understand the effect of evidence given sufficiently to make an informed judgment is taxed to the very utmost, and I can imagine it being exceeded.
The analytical starting point in a testamentary capacity claim is the English law principle of testamentary freedom, which as explained in Banks v Goodfellow , is that, ‘English law leaves everything to the unfettered discretion of the testator, on the assumption that, though in some instances, caprice, or passion, or the power of new ties, or artful contrivance, or sinister influence, may lead to the neglect of claims that ought to be attended to, yet, the instincts, affections, and common sentiments of mankind may be safely trusted to secure, on the whole, a better disposition of the property of the dead, and one more accurately adjusted to the requirements of each particular case, than could be obtained through a distribution prescribed by the stereotyped and inflexible rules of a general law.’
An eccentric disposition of property is not in itself evidence of incapacity, and it is the whole picture that needs to be looked at. Whilst T may make a valid will disinheriting his children out of capriciousness, frivolous, mean or even bad motives, and it is not the function of the court to substitute its own view of what T should have done, it does not follow that the court should not look for a justification for a change in T’s will or inquire why T disinherited a child.
‘An irrational, unjust and unfair will must be upheld if [T] had the capacity to make a rational, just and fair one, but it could not be upheld if he did not. It followed that the court must inquire why [T] has disinherited his children [i.e. what T’s reasons were] where there is a possibility that it is due to disease of the mind… the justice or otherwise of [T] excluding his daughters must as a matter of common sense have a bearing and cannot be excluded from consideration… provided that the inquiry is directed to [T’s] soundness of mind, and not to general questions of perceived morality.’ Re Ritchie .
Where a will has been drafted by an experienced independent lawyer who formed the opinion from a meeting or meetings with the testator that the testator understood what he was doing, a court will only set the will aside on the clearest evidence of lack of mental capacity.
The Court should be cautious about acting on the basis of evidence of lack of capacity given by a medical expert after the event, particularly when that expert has neither met nor medically examined the testator, and particularly in circumstances when that expert accepts that the testator understood:
(i) that he was making a will; and
(ii) the extent of his property.
Expert medical evidence will not necessarily outweigh the factual evidence of lay witnesses who had opportunities for observation and knowledge of the testator.