Template:M intro crime expert evidence

Expert evidence
(n.)

Evidence of a technical or scientific nature given by persons who did not necessarily witness events, but who have the technical expertise to explain the evidence and contextualise it for the jury.

Generally under rules of English court procedure, witnesses can testify as to what they see or hear, but may not offer an opinion as to what, in the context of the trial, their evidence means. Witnesses may not draw conclusions or make inferences from what they have seen: that is the jury’s job, under the direction of the judge.

As we know there are two kinds of evidence: direct evidence that testifies directly to an ingredient of the offence:

Fred saw Jack strike Jill.

And circumstantial evidence, which tends to make an ingredient of the offence more or less likely:

Fred saw Jack leaving Jill’s room with a knuckleduster.

If Fred only saw Jack leaving Jill’s room with a knuckleduster; he may not offer his opinion that therefore Jack struck Jill. That is an inference.

Experts as an exception to the rule

There is one exception to the rule against opinion evidence: where the facts to be put before the court are so technical that understanding them requires expertise the jury is not likely to have.

Here the Court may allow “expert evidence” to explain those technical details to the jury and the appropriate inferences that they might draw from them. Expert witnesses are not meant to take sides, but rather to assist the court in even-handedly explaining technical details to the jury. As such, they may offer opinions and draw conclusions on the technical matters for which they are qualified.

“The ultimate issue”

A rule forbidding experts from opining on the “ultimate issue” — that is, whether the defendant is or is not guilty — has in recent years been relaxed, possibly because an expert is never asked to opine directly on that question, it being a legal question, not a scientific one, in any case.

The theory was that this would “usurp the function of the jury of the ultimate trier of fact”.

An argument has been advanced, for example, that the CPS nixed a police inquiry into statistical evidence about the likelihood of Ms. Letby’s presence at every offence because it would offend this “ultimate issue” proscription.

We don’t know whether that was their actual basis for shutting down the inquiry because the CPS has claimed litigation privilege and declined to reveal its grounds. But it does not seem right: statistics can only go so far. They can provide a basis on which a reasonable jury could make that inference — as all circumstantial evidence does, they could help the jury to that conclusion — but the statistics cannot, of themselves, force that conclusion. The jury still has some work to do. It must decide what these statistics mean.

That all the collapses happened on her shift might be highly inconsistent with that being random, but there are many non-random explanations, and only one of them is that “Ms. Letby is a murderer”.

Statistics cannot rule out, for example, that Ms. Letby was “framed” for the murders by someone else. The jury alone can draw that inference: a statistician will not help them. Ergo: statistics do not themselves “prove” the ultimate issue. This is, presumably, why Dr Roy Meadow could cite implicating statistics in Sally Clark’s case.

Appealability

Now a quick technical sidebar about appeals in criminal cases: one can fairly easily appeal “matters of law” that come up during the trial: not just the judge’s final legal verdict, but directions she gives before and during the trial such as decisions on whether evidence is admissible, and whether the legal examination of witnesses was appropriate: the rules of criminal procedure are arcane and fiendishly complicated, and there is much hay to be made here.

It is much harder to appeal “findings of fact”: these are the jury’s opinions about the evidence presented to it, and the inferences and conclusions that it draws as a result. The jury’s deliberation is, more or less, sacred: jurors are sworn to lifelong confidentiality about the content of their deliberations on pain of contempt of court. There are many reasons for this stance, but the main one is that the jury’s rationale is deliberately opaque: it hears weeks or months of evidence, distils all of it to a “guilty” or “not guilty” verdict. It does not provide any rationale. No-one knows what the jury thought about the evidence, what evidence it relied upon, or what its rationale was for conviction or acquittal. It may even have little to do with the evidence at all. A jury might entertain acquitting a defendant it believed most likely to be guilty because it was dissatisfied with the police’s conduct of the investigation or evidence that emerged at trial.

In any case, appeal courts are most reluctant to second guess a jury’s assessment of facts and will only do so where, on the evidence, the jury’s decision is manifestly perverse.

The curious position of expert evidence

This makes speculative expert evidence — which will contain inferences and opinions that normal evidence cannot — peculiarly vulnerable to uncorrectable injustice in a criminal case.

How experts are engaged in English criminal trials has been the subject of ongoing criticism and debate.

Too much expert opinion evidence is admitted without adequate scrutiny because no clear test is being applied to determine whether the evidence is sufficiently reliable to be admitted.

This problem is exacerbated in two ways:

First, because expert evidence (particularly scientific evidence) will often be technical and complex, jurors will understandably lack the experience to be able to assess the reliability of such evidence.

There is a danger that they may simply defer to the opinion of the specialist who has been called to provide expert evidence.

Secondly, in the absence of a clear legal test to ensure the reliability of expert evidence, advocates do not always cross-examine experts effectively to reveal potential flaws in the experts’ methodology, data and reasoning.

Juries may therefore be reaching conclusions on the basis of unreliable evidence. This conclusion is confirmed by a number of miscarriages of justice in recent years.

Experts are engaged and paid for not by the court, but by the litigants who call them. Their evidence is thereafter subject to the ordinary adversarial process, primarily tested by cross-examined. Each side will do its damnedest to undermine the credibility of the opposition’s experts.

Cross-examination as a “laissez-faire leveller” makes some sense where witnesses are called by, and therefore presumed to support, one side or the other. But explicitly, expert witnesses are not meant to take sides: they are expected to provide “impartial, unbiased, objective evidence on the matters within their field of expertise”. Furthermore, the evidence they give is by definition technical and complicated. They are there specifically to help the jury to understand it. Having opposing experts going hammer-and-tongs at each other, and asking the jury to arbitrate between conflicting experts is hardly likely to help.

Whither an expert to support the proposition, “it wasn’t me, Guv”?

Nor does the intellectual premise of a criminal trial give the same scope for prosecution and defence to produce expert evidence. It is not as though both sides must prove their argument and whoever is more persuasive wins. Rather, generally, the prosecution must prove its case outright. That is all. The defence need not prove anything.

In most respects, this is to the defence’s advantage: when it comes to briefing experts, it is not. The Crown must have a clear, coherent theory of its case. It must prove every element beyond a reasonable doubt. It must establish the provenance of DNA, it must analyse fingerprints on the rifle, identify the substance splashed under the dashboard, explain the physical manifestations of air embolus, or calculate the air-speed velocities of unladen swallows. These are all matters that ask for technical expertise. As long as it can pour enough cold water on the prosecution’s efforts — which generally one does by cross-examination — the defence need not prove anything.

On protestething too much

Since it is not proving anything, the defence has less call to introduce its own expert evidence. What kind of expert could assist with the argument, “It wasn’t me, I wasn’t there, I had nothing to do with it, I have no idea what happened”?

One premise for a defence expert would be specifically to rebut the prosecution’s expert evidence. But there is jeopardy here either way.

On one hand, there is a fairly common perception among criminal defence lawyers that calling competing expert witnesses can sometimes backfire. It could come across to the jury as trying too hard to artificially manufacture doubt through a tendentious “battle of experts”. And experts can be hesitant directly to contradict their colleagues, whether out of common courtesy, professional dynamics, concern for their own reputations (few professionals like to be considered agitators within their own field) such that many experts — good ones, at any rate — may openly acknowledge scope for differing valid interpretations rather than contradicting a colleague. This is unlikely to be the knock-out blow a defence counsel might wish for. Cross-examination is a much more devastating tool. There are no niceties.

On the other hand, if prosecution evidence is not rebutted, it is the only evidence on the record when the jury retires to consider its verdict. Cross-examination can undermine a prosecution witness’s credibility, but still does not represent evidence in itself. It is not summarised and presented to the jury.

Lucy Letby as a difficult case

Lucy Letby’s case is unusual in that it hinges on expert evidence. There is no direct evidence of criminal behaviour at all, by anyone. The technical evidence relating to exogenous insulin and post-mortem symptoms of air embolus, is purely circumstantial. Beyond a notorious shift rota, the residual evidence specifically implicating Ms. Letby is strikingly weak.

JC has been critical of the defence strategy and not leading expert evidence, but on reflection, I wonder what good it would have done. If so, the fault lies here not with the defence but the process itself. You can find a more detailed bloviation on the topic here.

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If your case is, as loosely Letby’s case was, that no crime has been committed, then the question is, what would defence experts evidence seek to establish? They do not have an alternative theory of the case. Their position is that there is no case.

Plainly, an expert who is engaged and paid by a party to the litigation, but who is meant to be an impartial advisor to the court, is conflicted: She has a direct financial incentive to produce an opinion that her principal think is worth paying for. No defence team will pay an expert whose evidence will implicate their client. Nor will the crown court experts who exonerate the defendant. Expert evidence that is equivocal will not be of any great use in the cauldron of cross-examination.

Many experts do not like becoming embroiled in criminal matters: it is distracting to their own work, uneconomic and can have adverse consequences on their professional reputations and indeed livelihoods: a freelance specialist who testifies against the NHS hand that, in other respects, feeds her, may fear her regular income will dry up.

The error is to treat experts as part of the same adversarial process as ordinary witnesses. Explicitly, they are there to impartially assist the court and the jury in drawing appropriate inferences. The question for the court should be whether the inferences they ask the court to draw are reasonable. This can be done in camera, by consent between the parties. Barristers are ancient professionals bound by time-honoured ethical standards. They can be expected to act in good faith. It ought to be possible, in most cases, for prosecution and defence to produce a consensual statement of the expert evidence which can be presented to the jury without challenge.

Of course, the possibility remains that they cannot. There will be edge cases. But this debate need not, and absolutely should not, be conducted before the jury of laypersons. Presuming the jury has the analytical skills to assess the veracity of evidence given by experts who were only engaged because the jury did not otherwise have the analytical skills to assess the veracity of the evidence is a logical cul-de-sac. It makes no sense at all.

Solutionising for a moment, disputes could instead be resolved by the judge: the court can rule, as an inherently appealable matter of law, that one or other expert’s account will be put into evidence or that neither of the experts in sufficiently credible, or the subject matter is too speculative, so the expert evidence may not be introduced at all.