Lucy Letby

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Peasant: We’ve found a witch. May we burn her?

Bedevere: How do you know she is a witch?

Peasant: She looks like one.

Bedevere: Bring her forward.

A Woman with a parsnip strapped to her nose is thrust forward.

Woman: I am not a witch.

Bedevere: But you are dressed as one?

Woman: They dressed me up like this.

Woman: And this isn’t my nose! It’s a false one!

Bedevere: (To the peasants) Well?

Peasant: Well, we did do the nose. And the hat. But she is a witch.

While Sir Bedevere leads the crowd through the logical ways of telling whether she is a witch, enter King Arthur, as yet unobserved.

Bedevere: Does wood sink in water?

Peasant: No. It floats. (Pauses) Throw her into the pond!

Bedevere: (Patiently) What also floats in water?

Assorted Peasants: Bread! Apples! Very small rocks! Cider! Cherries! Gravy! Mud! Churches! Lead!

Arthur: A duck.

Everyone turns in awe and wonder to the King

Bedevere: Exactly. So, logically — ?

Peasant: (Slowly nutting it out) If ... she weighs the same as a duck ... she’s made of ... wood?

Bedevere: And, therefore?

There is a long, pregnant pause.

Peasant: A witch!

— Monty Python and the Holy Grail

Preface

This piece took a long time to write. JC is no criminal lawyer, has no particular axe to grind, and found himself constantly wondering whether he had been afflicted by his own tunnel vision. Had he too fallen into the oubliette?

Ms. Letby may have committed these awful crimes, or some of them, or she may be innocent. No one, but Ms. Letby, can know for sure. That is the dilemma: how to treat such uncertainty fairly. The question is whether the criminal justice system has done so. In light of its recent performances — Andy Malkinson and the Post Office sub-postmasters refer — we should hesitate before saying, “Well, she’s had her day in court and has been convicted” as if that is really the end of it.

The case raises fascinating issues that should interest lawyers in any discipline — and informed citizens in any walk of life, for that matter. Given JC’s general aspiration to try to understand the world as a complex system that behaves, and incentivises individuals to behave, in ways we would not expect, it seems like a fair topic for conversation at least.

On herd minds, groupthink and narrative bias

Herculio: ’Tis neither malice, spite, nor virtue
Whose ledger swells, or plucks, the seedy fruits of progress —
But mainly accident.
Lest thee with surety know aught else —
Withhold thy assignations.

Triago: Pish upon thee, Nuncle. Pish!
Dost thou mean to say
Things peel this way
Through doughty misadventure?

Herculio: Peradventure —

Triago: Pish abeam!
Has thou no more to say than that?
Wouldst thou on this shaky surmise
Withhold rebuke?

Herculio: Perchance, per case, mayhap dear Triago
’Twas but a fluke?

Triago: O! This nuisant planet weighs upon my soul!

Herculio: If ’tis this and nought beside
That flies you to a vernal rage
Our fickle globe in its manifold confound’ry
Lies prettily indeed
For thy alignment.

Lucy Letby is back in the news in 2024. On 24 May, the Court of Appeal denied her leave to appeal against her convictions. On 2 July, she was found guilty on retrial for the attempted murder of “Child K”.

All of this came against a backdrop wherein, on 13 May, New Yorker magazine published Rachel Aviv’s “A British Nurse Was Found Guilty of Killing Seven Babies. Did she do it?”, a 13,000-word investigative piece questioning Ms. Letby’s original convictions. This, in turn, followed a dogged campaign by a rag-tag bunch of amateur statisticians, scientists, forensic specialists and plain old keyboard warriors who were convinced of an injustice.

Since then, the mainstream media has leapt into the fray, most expressing rising discomfort about the safety of her convictions and some — notably the Daily Mail — backing both sides of the argument, running pieces in the same week by Liz Hull, who sat through the trial and is certain that Ms. Letby is guilty, and Peter Hitchens who is, vocally, not.

Having been convicted, in two separate trials, of multiple infant murders, the public’s view is — or perhaps was — clear: justice has been done. The sooner Ms. Letby’s name fades from the commonplace the better. As media consensus evolves the received view may fragment.

For as we look closer we see the picture is complicated. The physical evidence implicating Ms. Letby is all circumstantial: eyewitness evidence does little more than put her at the scene of the alleged incidents, and this may not say very much: it was a place she was, after all, contractually obliged to be.

Nor did anyone see Ms. Letby do any of the acts of which she has been accused. The medical evidence indicating foul play is in all cases technical and in most heavily conjectural. Some of the stronger parts of it — arguments about just how strong are heated — relate to babies who survived.

The evidence that resonates most strongly with the public — in particular, the notorious “shift rota” putting Ms. Letby at the scene of every incident and her alleged “confessional note”, written some months after her arrest — is not strong at all.

Nor is the evidence that any infants were the victims of foul play at all, overwhelming. They were, after all, premature infants in an intensive care unit. If they were not at high risk of mortality, they would not have been there.

The case for conviction thus emerges from a preponderance of small pieces of evidence that all point towards Ms. Letby, but weakly. There is no smoking gun. The most compelling proposition seems to be Ms. Letby’s opportunity: she alone was rostered on and onsite during all the alleged incidents.

The circumstantial evidence of the shift rota is founded on improbability. What are the odds that the one person present at all the incidents did not have something to do with them? Neither prosecution nor defence framed the argument in terms of probabilities, and none brought any statistical analysis to bear on the question. This, as several statisticians have noted, is a pity. Especially given the criminal law’s chequered history with statistical reasoning.

A binary status

Now she is in prison for the whole of her life, all appeals exhausted and all pending retrials completed, just two narratives remain. The simple one is that Ms. Letby is the personification of unspeakable evil: a serial murderer of defenceless, premature infants. The other one is that she is the victim of a breathtaking, and no less unspeakable, miscarriage of justice.

Advocates on both sides grow increasingly entrenched.

But on the very same probabilities, this is a most unlikely place for the public debate to have landed.

Serial murderers of any kind are vanishingly rare in Britain. Wikipedia lists fifty-five, since 1600. But so are miscarriages of justice. Wikipedia lists fifty-four, since 1255.

Both narratives are highly improbable. Of all the explanations we might offer for the unusual spike of incidents in 2015 and 2016, as a matter of “prior probabilities” they are the least plausible: all else being equal, it is highly unlikely there was a serial murderer at work in the Countess of Chester Hospital. But, one having been convicted, all else being equal it is highly unlikely her conviction was a miscarriage of justice. This is why tempers run high. Both positions seem untenable.

They leave untouched a wide range of more probable, but less satisfactory explanations. We like our narratives to explain the world to us. We do not find useful ones that say, “Well, we just don’t know”. We prefer conclusions, however unlikely they may be.

So our institutions persuade us to deprive Ms. Letby of that wide middle ground in which she was an ordinary person, with pluses, minuses, virtues and failings. Neither angel nor devil: she just had the misfortune to be involved, somehow — we know not how — in these tragic events.

Especially where, as here, there is no direct evidence, the criminal justice system forces us into an odd kind of anecdotal reductionism: we must focus on discrete actions and isolated pieces of highly technical evidence: literally, microscopic observations: implied levels of insulin and c-peptide; unusual skin blotching patterns and so on. We expected to thread these atoms together and be able to deduce from them a compelling picture of emergent human, moral actions.

We are pattern-matching machines. We use whatever morsels we find to build a narrative. Once we have a theory, we dive deeper into the details to prove it out. We strive to fit the remaining facts into our model. We are therefore at great risk of tunnel vision, when our immersion in those atomic details crowds out a wider perspective.

Only later, if at all, do we ask the “big picture” question: leaving aside the microscopic details how plausible is it that this was murder?

Case for the prosecution

Lucy Letby’s case, on its face, seems a lot more like Lindy Chamberlain’s than David Bain’s. It is not beyond question that none of the infants — all extremely premature and in an intensive care unit, after all — met with foul play. For each event taken separately, there is a plausible non-criminal explanation. Indeed, Ms. Letby was not immediately suspected of involvement in any of the deaths.

The case for Ms. Letby’s innocence has been well-stated enough. The New Yorker piece describes the statistical problems. Peter Elston and Michael McConville’s We need to talk about Lucy Letby podcast goes into great detail about the credibility of the technical medical case — it is not very credible — and the excellent TriedbyStats website does a tremendous job of conveying just how unintuitive the statistical implications of the evidence are.

Of course, individuals committed to the proposition that there has been a miscarriage of justice may suffer their own form of “defender’s” tunnel vision — but in the main, their arguments are clear, well-grounded and compelling.

Yet this was no kangaroo court: the British justice system is rightly a thing of international renown. Over centuries, the common law has developed principles that avoid injustice. Clearly, they are not perfect, but the presumption of innocence, the burden and standard of proof, rules of criminal procedure regulating admissible evidence, pre-trial disclosure and acceptable methods of examination and cross-examination are specifically designed to ensure fairness. Ms. Letby certainly got her day in court — ten months worth of them — and spent 14 days giving evidence during her trial, at which she was represented by a King’s Counsel who left few stones unturned.

Still, there ought to be clear, well-grounded and compelling arguments to displace the presumption that Ms. Letby was simply unlucky, implicated by circumstantial evidence but fundamentally just in the wrong place at the wrong time, and instead set her out as a demonstrable serial murderer.

We might expect tunnel-visioned innocence campaigners to overlook inconvenient facts — half of confirmation bias is blindness to what you do not want to see — but not those who are persuaded of Ms. Letby’s guilt. They must, surely, be able to articulate these “clinchers”.

A strident segment of the commentariat is persuaded of her guilt. A Reddit community of yeomen amateur investigators numbers 11,000 members and is replete with theories but brooks no sympathy. Journalists who covered the trial in detail have published films, podcasts and have forthcoming books about the “Killer Nurse”. The prosecution’s main expert witness, Dr. Dewi Evans, has gone to unusual lengths to dismiss the concerns of innocence campaigners whom he feels to be unfairly targeting him.

But if these people can articulate these damning facts, in their voluminous reporting to date, they haven’t, particularly well.

The timid voice of protest grows louder.

Confirmation bias

“Angel” and “devil” narratives become self-fulfilling: once you’ve adopted one, you can panel-beat almost any subsequent information to suit your view.

The little definitive peripheral evidence there is about Ms. Letby gets coloured dramatically by whomever is looking at it. For example, her social media activity. Here is BBC Panorama reporter Judith Moritz, in a piece to camera:

“Sparky, full of fun, popular — she looks like the life and soul of the party in these photos. I don’t know what Britain’s most prolific child killer should look like — I’m pretty sure it’s not this, though.

And then, a few moments later:

“She comes across as — mousy; a bit normal — you can’t really marry that with the enormity of what she’s been accused of.”

If and only if we are persuaded of her guilt, should an outwardly vivacious personality and busy social life mark her out as a monster. In that case it corroborates and amplifies her wickedness.

But it cannot also be evidence of her wickedness.

For, if we are prepared to give Ms. Letby the benefit of the doubt, this is perfectly normal behaviour. It tells us nothing: it places Ms. Letby in that ordinary space, with the rest of us, within a standard deviation of the mean. There have been very, very few serial killer nurses. We have no idea what, as a rule, they are like. This information does not help us. This simply tells us that Ms. Letby is not unusual.

That Ms. Letby searched online for the parents of the deceased is consistent with either breathtaking malevolence — if you take it that she is a serial killer — or affecting compassion — if you take it that she is not. By itself, it is evidence of neither. We all search online for individuals we meet in real life — even people we know we probably shouldn’t: this is perfectly normal behaviour. We are curious, imperfect animals.

Doubt

Whatever the reason for it, where there is no direct evidence of a defendant’s foul play — of any foul play — there will always be some doubt. No one saw anything. Circumstantial evidence does not speak for itself. Inference is needed. An inference — from this observation to that conclusion — is necessarily an imaginative act. There are good inferences and bad ones. The question will be whether a given inference can be justified in all the circumstances. Has the prosecution done enough to remove all reasonable doubt?

In some types of crime — burglary, say — we should expect no direct evidence: competent burglars target unmonitored environments where there are no witnesses.

But burglars, we fancy, would steer well clear of the intensive care unit in a neonatal hospital. There could hardly be a worse environment to get away with crime. Entries and exits are monitored and audited. Medicine is logged, controlled, secured and signed in and out. Medical experts conduct routine rounds and check on patients around the clock. Biochemistry is complex and its signals are delicate. There are specialists at hand with deep expertise and sophisticated machinery who can detect the merest traces of the unusual or sinister.

No direct evidence

So a lack of direct evidence is a curious feature of the “healthcare serial murder” cases. This one is no exception. No one saw anyone actually doing any harm. The circumstantial evidence tends to be consistent with, rather than indicative of, wrongdoing.

Avoiding eyewitnesses and leaving no forensic evidence would require great skill, caution and planning. You would expect a malign actor to refine and perfect a technique — a modus operandi — and stick with it. You would not expect such a person to make opportunistic attacks, use multiple, unrelated methods or improvise on the spot to suit the circumstances and go completely unnoticed.

Certainly not one under active suspicion. Hospital consultants raised concerns with management about Ms. Letby as early as October 2015, seven months before she was eventually suspended. You would expect some level of elevated monitoring, even informally, after that.

Calculated or opportunistic?

Ms. Letby is alleged, variously, to have injected air intravenously and via nasogastric tube, caused blunt force trauma to internal organs without external injury (quite how is not clear), overfed, dislodged breathing tubes, poisoned with insulin, and physically attacked her victims.

Yet despite these attacks using multiple, unrelated methods and sometimes being opportunistic and improvised, Ms. Letby still left no implicating evidence.

What is more, the court turned this implausible variability on its head, describing Ms. Letby as therefore “premeditated, calculating and cunning”, using a number of different methods, thereby misleading clinicians into believing the collapses had a natural cause.

Perhaps — or maybe the collapses did have a natural cause. For there is one other germane feature of an intensive care unit: its patients, by unfortunate necessity, tend to be very, very sick. They have an unusually high probability of dying. If they did not, they would not be there.

There is a benign alternative explanation for this “cunning calculatedness”, therefore: that it was nothing of the kind.

Burden of proof

The law requires criminality, not innocence, to be proven. The burden is therefore on the prosecution, and it is heavy: it must prove its case beyond reasonable doubt. This is one way the system is set up to avoid injustice.

Between prosecution and defence, then, it is not a “fair fight”: the defence has a much lower threshold to win its case. If the defence must prove anything — technically it need not, but it would be a brave defendant who did not present some alternative theory of the case — it is simply that there is some credible doubt that what the prosecution alleges is what actually happened.

When a seriously ill patient dies in intensive care, even unexpectedly, and no direct evidence — or circumstantial evidence, indeed, beyond mere presence — implicates the defendant, there is credible doubt.

For each collapse at the Countess of Chester Hospital, taken in isolation, there was reasonable doubt that the victim met with foul play at all, let alone that Ms. Letby was responsible for it. We know this because pathologists originally attributed every incident for which Ms. Letby was subsequently charged to natural causes.

So, how can a series individual cases, all of which do have reasonable doubt, turn into a single collection of cases which do not?

The answer is probability.

Probability and the shift data spreadsheet

Once is happenstance. Twice is coincidence. Three times is enemy action.

—Ian Fleming, Goldfinger

It is sometimes said that Ms. Letby was “not tried by statistics”, the implication being that, because they were not argued at trial, probabilities did not influence her conviction. But this is not right: a case depending on circumstantial evidence is necessarily about probabilities. The “probative value” of circumstantial evidence is simply this: “Does this evidence make it more or less likely that an event, which no one witnessed, happened?”

Statistics are not always appropriate for assessing probabilities, especially where data are limited or collected in unusual circumstances. But the shift data are statistics. The Crown may not have framed its arguments about them that way, but they were statistical: the shift data had no probative value other than as statistics.

Now: flip a fair coin once and heads is as likely as tails. That’s “happenstance”.

Flip it twice and, while two heads are less likely, they still carry a one-in-four chance. That’s “coincidence”.

But flip it twenty-five times? There is a less than one in thirty million chance of getting twenty-five consecutive heads. If you do, it is most unlikely your coin is “fair”.

The shift data seem to be like a string of twenty-five coin flips that all came up heads. If they are they can dispel those “credible doubts” that remain in individual cases: okay, each single collapse may have been as likely as not innocent. Two, a coincidence. But twenty-five?

If there is a single clinching argument in the evidence presented against Ms. Letby, this was surely it.

But there are two objections to this reasoning:

Firstly, it draws a false dichotomy between “foul play” and “natural causes”.

The range of “natural causes” that could trigger a medical collapse is wide and necessarily indeterminate. The range of malicious causes of a collapse with which a defendant has been specifically charged are necessarily limited and finite. It is not a “fifty-fifty” alternative. Since Q.E.D., we don’t know them all, we cannot calculate prior probabilities of an unlimited range of potential natural causes. Nor, therefore, can we calculate their inverse, which is the probability of the single malicious cause. This puts an upper bound on the prior probability of that malicious cause and ought to make arguing malice from simple probabilities very difficult.

Secondly, the logic only holds if the “flips” really were consecutive.

If there were fifty flips, and only twenty-five came up heads, that would not be unusual at all.

And here we come to the notorious spreadsheet setting out the shift data. It comprises a small selection taken from a much bigger sample of a thousand or more shifts in the thirteen-month “suspect period”. It contains only the shifts germane to the events for which Ms. Letby was charged. Given the prosecution’s reliance on circumstantial evidence to bring charges, we can assume that “Ms. Letby being on duty for a given incident” determined whether she was charged with it. If she was not, there would not be enough evidence to dispel reasonable doubts.

That is to say, all the events on the spreadsheet necessarily occurred while Ms. Letby was on duty. The chart tells us simply that she was on duty every time she was on duty.

There is some debate about whether, when they originally reviewed the hospital data, the police already had Ms. Letby in mind as a possible suspect. If they did not, it is argued, that might give more weight to this evidence. But the material point in time is not when they first reviewed the evidence but when they assembled this spreadsheet. At that point, in preparation for the trial, the prosecution certainly did have Ms. Letby in mind, so the question is moot, and in any case, could have been avoided by providing all relevant shift data to the court, rather than a conveniently filtered subset.

The spreadsheet’s use in evidence has thus been widely criticised, perhaps most eloquently by Rachel Aviv in her New Yorker article. Aviv raises the prospect of the “Texas sharpshooter” fallacy. Are all the incidents included? Did any relevant events happen when Ms. Letby was not on duty? What even counts as a “relevant event”? Has inconvenient data that does not fit the prosecution narrative been omitted?

There were ten other deaths in the suspect period for which Ms. Letby was not charged. We do not know why she was not charged, nor whether she was on duty for these events. (A few commentators claim to have seen evidence that Ms. Letby was on duty for every death in the suspect period — but that evidence is not public, and importantly does not seem to have been presented at trial.)

Had the Crown submitted a comprehensive table including every shift in the suspect period, and marked every incident on it, whether Ms. Letby was implicated or not, we might have a better story. But they did not.

In the aftermath of Ms. Letby’s original conviction police hinted “further charges may be forthcoming” as they continued to review thousands of historical cases dating back to 2012. That was easy to say and hard to rebut. But, in the twelve months since that statement, the police have brought no further charges. Since they have now had nine years to review that evidence, it does not seem unreasonable to suppose the search has not turned anything up.

On “tonewood” and prosecutor’s tunnel vision

Triago: Ho, Ho.
Let not thy witty fool, nor his foolish wit
Besmirch the fruited science of th’ academy.
“A little learning is a dangerous thing” —

Nuncle: Yet not half so dangerous as a lot.

Triago: — So sayeth Pope, you know.

Nuncle: But not the one in Rome:

Queen: Good ser knight: art thou drunk upon the Pierian spring:
A hypoxic draft that suffocates the brain,
So deep no shaft of light can bring it round again?

Triago: My conjecture comports a grain of truth
As pure and true and golden—

Nuncle: — but yet no more roundly causative
Than are the month-past flappings of a Latin papillon
Upon a brewing Filipino typhoon.

—Otto Büchstein, Die Schweizer Heulsuse

JC has, elsewhere, waxed long and lyrical about the collection of cognitive biases called prosecutor’s tunnel vision. They tend to show up where clinching evidence does not: if there were any clinchers, things would be clinched. Those prosecuting — and, for that matter, defending — fixate on finer and finer technical details to win more and more isolated arguments. Once one side goes down a rabbit hole, the other is obliged to follow. Insulin assay tests, entry card swipe data, the significance of skin discolouration — all have been cited as smoking guns for prosecution and defence when they are transparently nothing of the sort.

The fact is, there are very few political, social, and especially personal problems that arise because of insufficient information. Nonetheless, as incomprehensible problems mount, as the concept of progress fades, as meaning itself becomes suspect, the Technopolist stands firm in believing that what the world needs is yet more information.

—Neil Postman

Information is fractal. The more you look at it, the more you can create: it subdivides infinitely — there is no bottom — and the more raw information you create, the more possible arguments you can have about it. This is the implication of information theory: the more data points there are, the more unique patterns you can make from them. If we take it that a theory is simply “a pattern of consistent data drawn from available information from which we can draw a valid inference” then the more information you have, the more plausible alternative theories of the case you can make.

In other words, you cannot win an argument by simply descending further into the weeds. Those who have drunkenly debated evangelical Christians, resolute atheists, real ale connoisseurs, or tonewood freaks — JC has done all of these — will know this. The thing about descending into the oubliette is that as the points atomise the arguments grow more heated and numerous and the less difference they can possibly make.

This is the “tone-wood” debate in a nutshell: sure, in theory the harmonic resonance of nitrocellulose varnish could affect the sound of an electric guitar, but not so as a mere human could possibly hear it in perfect conditions let alone in the moshpit at the Roxy on a sweaty night in 1976. Whatever made Steve Jones’ Les Paul sound like a screaming chainsaw, that is, it wasn’t the varnish.

Zeroing in to draw further inferences will not help. The only way to do it is by zooming out.

Oh come on, ref —

“In this case, Letby is a young, white, English nurse from a reputable, normal background. So it’s not surprising that some people respond to the fact that she has been found guilty of killing babies by saying it hasn’t happened. It becomes about shooting the messenger — which, in this case, is me.”

—Dr. Dewi Evans, prosecution expert witness, The Times, 7 July 2024

It is a common enough trajectory: we start with an instinct that a popular, diligent, young, female nurse from a stable background is the last person you would expect to be a serial murderer.

We quietly chide ourselves about our unconscious bias in favour of people like us. We remind ourselves that natural sympathy with our in-group is not scientific, let alone legal evidence. The law requires us to set that kind of pre-judgment aside and consider the evidence as it accumulates in the abstract and on its merits.

But hold on: if circumstantial evidence “changes the prior probability of an event”, it adjusts the inferences we can reasonably draw from baseline facts.

Inasmuch as Ms. Letby’s background, socialisation and mental health reflect the prior probability that she would not be a murderer, and inasmuch as they are not displaced by direct evidence that she is a murderer, then our intuitions are useful. They are all we have. If we ignore them, our pattern-making instincts may lead us down rabbit-holes, chasing down tendentious circumstantial minutiae at the expense of basic common sense.

This is how miscarriages of justice happen.

The Northern Territory Coroner’s report to Lindy Chamberlain’s trial is perhaps a high watermark of this kind of thing. Recall that Lindy Chamberlain had immediately claimed to have seen a dingo exiting Azaria’s tent. A week after her disappearance, Azaria’s torn and bloodstained jumpsuit was discovered in the desert, 4 km from the campsite. Case closed, surely?

No. Rather than seeing this as corroborating Mrs. Chamberlain’s story, the coroner arrived at precisely the opposite conclusion:

The evidence in relation to the clothing is consistent with an attempt to simulate a dingo attack on a child by person or persons who recovered the buried body, removed the clothing, damaged it by cutting, rubbed it in vegetation and deposited the clothes for later recovery. Such deposition is indicative that the deposition was made with the knowledge that dingos were in the area.

In addition, there is no evidence to positively support the involvement of a dingo in the taking of the child, the carrying of the body some four Kilometers and removing the body from the area where the clothes were found.

Prior probabilities, again: not only did Lindy Chamberlain have no opportunity to fabricate this evidence or make the eight-kilometre round trip to plant it, and not only did no one see her doing so, it was in any case vanishingly unlikely that a sane, well-adjusted and happily-married mother of three children would murder her own child in the gruesome way suggested. Without direct evidence to support the coroner’s theory, it was preposterous.

Before even entertaining the possibility of foul play, the coroner should have asked himself, and satisfactorily answered, a far simpler question: why on Earth would any normal, well-adjusted mother do that?

Now, barristers will remind us that motive is never required for a criminal conviction, and this is true. But. If there is no direct evidence of wrongdoing, there is a plausible innocent explanation and we cannot plausibly answer the question, “Why on Earth would she do that?” we should proceed very carefully.

Are we really to believe that an experienced nurse with no history of mental illness, criminality, sociopathy or familial dysfunction, let alone prior medical misadventure — could suddenly transform herself from a trusted senior medical professional into a killer of such sophistication as to go undetected for eighteen months? Even while suspected?

Are we really to believe that, when suspended from her duties as a result of management suspicion of criminal activity, a serial murderer would launch a grievance procedure to get back on the ward?

How could she do this? Why would she do it? Why, in the absence of compelling, hard evidence displacing the usual presumption that this is not what normally socialised people do, would the preferred theory of the case be serial murder, and not “an unusual but not remarkable cluster of accidents”?

Miscarriages of justice and the perfect crime

Lucy Letby may be guilty. These preposterous suppositions may in fact be true. But we may still question the process. It may seem outrageous that there is such a thing as the “perfect” crime, where a villain leaves no trace and walks away scot-free, but this is the cost of ensuring the system does not lightly condemn the innocent. As a general proposition, you cannot convict someone of a crime for which there is no evidence.

Here, again, is our old friend form over substance: this time, the requirement for form following the uncomfortable implication of epistemology that we cannot know substance. Form is all we have.

Here the difference between direct and circumstantial evidence is stark. If there is reliable witness evidence that the defendant did it, or did not do it, then the standard of proof can be satisfied easily. There is not much call, or even scope, for doubt.

But where there is no direct evidence that the defendant did anything — no eyewitness testimony, no CCTV, no uncoerced confession — then our inferences from circumstantial evidence that she did do something will have to do an awful lot of work.

They must create an overwhelming probability that these were not natural causes but murders and the defendant was responsible for them.

The healthcare serial murder cases

There is another baseline probability to consider: that there has been a miscarriage of justice perpetrated against a medical professional accused of serial murder by poisoning.

Here is the scenario:

There has been an unusual increase in incidents at a hospital for which no obvious “innocent” explanation exists. One nurse was present for all the incidents, but there is no direct evidence that she actually harmed any patients. Though many pieces of weak circumstantial evidence are consistent with the nurse’s involvement, she had no apparent motive, nor any history of violence, antisocial behaviour, criminality or mental illness. The nurse steadfastly denies responsibility.

This might sound like a highly tailored set of circumstances, specific to Ms. Letby alone.

But in the last fifty years, several cases have fitted this description exactly: Jessie McTavish (Scotland, 1974), Filipina Narciso and Leonora Perez (USA, 1975), Susan Nelles (Canada, 1981), Orville Lynn Majors (USA, 1993), Jane Bolding (USA, 1998), Daisuke Mori (Japan, 1999), Vickie Dawn Jackson (USA, 2000), Colin Norris (UK, 2002), Benjamin Geen (UK, 2004), Abraão José Bueno (Brazil, 2005), Petr Zelenka (Czech Republic, 2006), Lucia de Berk (Netherlands, 2010), Rebecca Leighton (UK, 2011), Victorino Chua (UK, 2011), Daniella Poggiali (Italy, 2014) and, of course, Ms. Letby.

Of these 17 prosecutions, two (Nelles and Leighton) were abandoned before trial, five (Filipina Narciso, Leonora Perez, Bolding, De Berk and Poggiali) were convicted but then substantively acquitted on retrial, and three (Geen, Norris and Letby) are the subject of active ongoing innocence campaigns.

This is not to suggest it is impossible for a nurse fitting Ms. Letby’s description to be guilty of serial murder, but that the baseline probability of a miscarriage of justice in a case like hers is surprisingly high.

To return to the initial observation: serial murders are vanishingly rare, healthcare serial murders even rarer and so, as a general rule, are miscarriages of justice.

But miscarriages of justice in healthcare serial murder cases are pretty common.

— So?

It remains to be seen whether the criminal justice system will allow any of this to be tested. Ms. Letby’s conventional avenues for appeal are already exhausted. Just as the system is designed to afford defendants a fair hearing before the jury’s decision, the prerogative of certainty in the criminal justice system makes it very hard for a defendant to challenge a jury’s decision on the facts afterwards.

Failures of Ms. Letby’s defensive strategy — not calling expert witnesses, not introducing evidence that was available at the time of the trial, not challenging the misleading probabilities implied by the shift rota — are not grounds for further review. This is, yes, form over substance, but the hurdles are still formidable.

Influential commentators have questioned whether the procedural rules surrounding the use of expert witnesses are fit for purpose in light of Ms. Letby’s case. There is some suggestion that key evidence was misinterpreted at the original trial. This might be grounds for appeal, but it does not seem likely.

The other unknown is the forthcoming Thirlwall Inquiry into the deaths at the Countess of Chester hospital. Its terms of reference do not include relitigating Ms. Letby’s convictions, but the rising pitch of public disquiet may make their adjustment unavoidable.

We will see.

Resources

Online

There is a lot of debate online about the case, much of it acrimonious and, even when not, it descends into deep oubliettes of peripheral materiality.

JC’s observation — perhaps evidence of his own tunnel vision — is that the arguments defending Ms. Letby tend to be more analytical and less emotionally involved than those favouring conviction. The sceptical case focuses on the soundness of the statistics and the consistency and plausibility of the forensic analysis (albeit this can descend into the oubliette at times). Those supporting conviction can be more emotive and ad hominem, describing ms. Letby’s defenders as “conspiracy theorists”, “cranks”, a “diverse band of fanatics and pseudo-scientists”, and “members of the ‘butterfly’ gang” who comprise “half a dozen or so middle-aged women who wear their yellow badges with pride” and who count, er, “scientists, neo-natal nurses, doctors and statisticians” amongst their number!

Arguments for the defence

• A British Nurse Was Found Guilty of Killing Seven Babies. Did She Do It? Rachel Aviv, New Yorker, 13 May 2024
• We need to talk about Lucy Letby. This podcast is a sceptical examination of the statistical and medical evidence by Peter Elston, a self-described “amateur statistician” and Michael McConville, a retired neonatal doctor.
• Mephitis - The Lucy Letby Case May In Time Be The Story Of The Century: Michael McConville’s website.
• TriedbyStats is a very cleverly constructed interactive website that brings the counterintuitive probabilities that underlie the case to life.
• Richard Gill statistics — Emeritus Professor Gill of mathematical statistics at Leiden University was instrumental in securing the acquittals of Lucia de Berk and Daniela Poggiali, and (perhaps for that reason) attracts quite a bit of vitriol from “prosecution” sources.
• Science on Trial: A website founded in September 2023 to examine predominantly the forensic medical evidence in the Letby case by Sarrita Adams, a biochemist based in San Francisco.
• The r/ScienceLucyLetby subreddit subreddit is generally open-minded but, focusing on the forensic and statistical evidence as it does, tending to support the sceptical case.

Arguments for the prosecution

• I helped convict Lucy Letby. Now her supporters are targeting me, Prosecution expert witness Dr. Dewi Evans, The Times, 7 July 2024
• Why the Lucy Letby conspiracy theorists are wrong, Liz Hull, Daily Mail, 6 July 2024
• Double Jeopardy - The Law and Politics Podcast episodes of 16 August 2024 (casting doubt on the doubters) and 24 August 2024 (somewhat hastily walking their position back after criticism of the earlier show)
• The Trial of Lucy Letby podcast (Liz Hull, Daily Mail)
• The r/LucyLetby subreddit is firmly pro-guilt, and brooks no dissent: those questioning the convictions (such as JC) are summarily blocked.

Similar difficult cases

There are excellent podcasts on each of the difficult cases mentioned in our “Prosecutor’s tunnel vision” article.

• A Perfect Storm: The True Story of the Chamberlains
• Conviction: The Christchurch Civic Creche Case
• Black Hands: A Family Mass Murder

See also

• Miscarriages of justice
• Texas sharpshooter
• Prosecutor’s tunnel vision