So begins The Investigation of Lucy Letby, Netflix’s inevitable true-crime take on the former nurse convicted of killing seven babies and attempting to murder seven more at the Countess of Chester Hospital. It is by now a story familiar to readers around the world: Letby, 36, is serving 15 whole-life terms for the murders. The blank-eyed mugshot from which most will remember her is used in the documentary’s title card, overlaid with infamous, supposedly confessional scrawlings on a Post-it note found among her things. This footage has itself been the subject of media furor in the past week, after Susan and John Letby told The Sunday Times that they were not aware producers were showing the inside of their home; watching the arrest back “would likely kill us”, they said. Netflix’s dramatic opening scene, grief cracking through the quiet of a suburban morning, “is a complete invasion of privacy”.

I doubt many viewers will see it as such. Few could feel anything beyond curiosity at this rare humanizing vision of a woman who has for so long been presented as a symbol of perfect evil. This gawky, lanky, weird lady, living with her parents in a pink-and-purple bedroom, is softly talked down to by young female officers who try to find her shoes. This is not a supervillain; this is a timid odd bod overwhelmed by her own gruesome actions, led away with a lump in her throat like a little girl frightened of going on a school trip. It’s painful and private, yes, and therefore madly compelling: how could this sort of person do that? The woman’s diary had a knitted dog stuck to the front, for god’s sake. On her bedroom wall, a framed motivational quote ordered the nurse to “leave sparkles wherever you go”.

Here lies the intrigue of Lucy Letby: completely unreadable, so banal yet so deranged, so weak and yet so predatory. Dr John Gibbs, an avuncular colleague, tells us in an interview from a hospital corridor that “there was nothing about her that made her stand out to me”. A mother, recalling Letby’s care of her baby “Zoe”, says that all the nurse did was stand with a clipboard, “watching us”. Later, in the courtroom, she was caught “staring” at her once again.

Letby is Shakespeare’s Iago plonked in an NHS ward in middle England: a “motiveless malignity”, as Coleridge put it. The villain of Othello dissolves upon discovery, robbing the audience of the satisfaction of understanding him: “Demand me nothing. What you know, you know.” To a perplexed public, Letby shares cunning Iago’s inscrutable id — a vacant, quiet girl with sly rot underneath. The filmmakers pointedly leave her final words as “no comment”; Letby is a true agent of chaos, appearing not to know herself why she does what she does. The fact that even the producers do not seem to understand her emboldens the Letby-truther camp: if this nurse had meant to kill, surely she’d have revealed, even inadvertently, a reason?

The more we know about Letby, the more she seemed like a woman possessed. The scribblings on that Post-it note were like the ramblings of a self-hating teenaged girl: “I am evil”; “I did this”; “I don’t deserve to live”; “I killed them on purpose”; “I am a horrible evil person”. “Horrible” is perhaps the most telling word Letby could have used: instead of swearing, she uses a limp playground insult, the kind that is growled by a grump in a Roald Dahl story. Even when appearing to confess to a string of infanticides, Letby is as weak as primary-school squash. In the moment when the viewer is meant to understand her, she dissolves: could this really be the “poisoner on the ward” mentioned by one police officer, the diabolical dame of a penny dreadful? Could this sad sack, for whom “horrible” is the deadliest swear and whose duvet set says “sweet dreams”, really have done this?

The motives proposed by prosecutors have been as follows: that Letby wanted the sympathy and attention of a male consultant on the ward; that she wanted to play God; that she hated herself and felt murderously inadequate; that she was thrilled by parents’ despair. Of these, the infatuation theory is the most exciting — after all, the unnamed alleged love interest prompted the first “flicker of emotion” from Letby, as The Independent put it, in the entire trial. Only text fragments support this hypothesis, with no direct proof.

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Yet instead of answering the question viewers really want to know — why? — Netflix’s producers focus on whether Letby could have done it at all. It’s a compelling case: statistical evidence of shift patterns misused; a medical paper misinterpreted by an allegedly unreliable expert witness; survival rates on a par with, rather than dwarfing, those in the rest of England and Wales. Letby’s great champion, the criminal barrister Mark McDonald, pours scorn on the trial and is somewhat convincing. The viewer is left with an itch of doubt: selling the documentary as an “unsolved mystery” rather than “the anatomy of a killer” is catnip to the truthers.

Letby has come to play a strange role in the British — and even the global — imagination. This is no Myra Hindley, a peroxide anti-Marilyn; if anything, Letby more closely resembles Grantham hospital’s own killer children’s nurse, “Angel of Death” Beverley Allitt, for whom Munchausen-by-proxy answered a lifetime of mediocrity and insecurity. But Letby, now the most prolific child serial killer in modern British history, eclipses Allitt precisely because of her tantalizing blankness, the ease with which we can project nightmares of putrefied femininity onto her long and sullen face.

It is far more frightening to imagine bloodlust in an everywoman — one who misses her two cats, Tigger and Smudge, and weeps in her girlish bedroom. Seven babies who never went home demand the unmasking of pure evil, rationalized and banged to rights and locked away for life. Letby denies us this catharsis. Instead she infects us with her own lingering insecurity — and the painful admission that sadism could be everywhere and in anyone.

UnHerd can now reveal that this was a statement to police by Dr Astha Soni, a pediatrician at the neonatal unit where Letby worked. And it raises serious doubts about the claims that Letby attempted to murder two babies by poisoning them with insulin. Yet the document was not disclosed to the defense before her first trial began, nor during her unsuccessful bid to appeal. Furthermore, we have discovered a second critical omission, involving the outbreak of a dangerous virus in Letby’s neonatal ward.

As Thirlwall’s inquiry proceeds, troubling concerns over Letby’s seven convictions for murder and eight for attempted murder — she is now serving 15 whole life sentences — have only swelled. The UK-based consultant neonatologists Svilena Dimitrova and Neil Aiton have submitted detailed reports on three of the babies’ deaths that refute the evidence given at Letby’s trial, and a separate panel of international experts led by Canadian neonatologist Professor Shoo Lee concluded last month there was “no medical evidence to support malfeasance” in the deaths Letby was convicted of causing. Their assessment: the babies died from “either natural causes or bad medical care”. It all points to a troubling question: is Lucy Letby truly guilty beyond reasonable doubt?

Dr Soni’s secret, undisclosed police statement concerns “Baby Y” — born just after midnight on 2 November 2015, some months into the rise in neonatal mortality at the Chester unit that the police were investigating. Letby was never formally accused of trying to harm him, but the prosecution experts who gave evidence at her trial believed she poisoned him with insulin, just as she did Baby F and Baby L, whom she was convicted of trying to murder. Since Soni’s statement undermines that analysis, it raises worrying questions about the experts’ evidence in those other cases.

Baby Y was a big baby, born two weeks after his due date following a long and difficult labor. He was finally delivered by an emergency cesarean, and transferred to the hospital’s neonatal unit early on 3 November. This happened, Soni said in her statement, after the baby experienced “dusky episodes”, struggling to breathe and turning blue. In the second of these episodes, Soni told police, the baby was not only blue but unconscious, with “his eyes rolled up” and his torso “floppy”. Once on the unit, he was given oxygen, but “didn’t pick up straightaway”. Half an hour later, Baby Y suffered what Soni called a “seizure” — turning blue and making “no respiratory effort”.

When Dr Soni saw the baby again later that night, she said, he was hypoglycemic. Despite increasing doses of dextrose, his blood sugar level remained stubbornly low. His diagnosis, Soni said, was “congenital hyperinsulinism” or CHI, a genetic condition in which babies produce excess insulin. This diagnosis was supported by endocrine experts at Alder Hey hospital in Liverpool whom the Chester doctors consulted, the statement went on. It is also possible that he was suffering from “transient hyperinsulinism”, a non-genetic, common condition with natural causes whose symptoms would have been identical. After more seizures Baby Y was given several drugs and slowly improved, but outpatient treatment for “persistent hypoglycemia associated with seizures” had to be continued for over seven months.

Medical experts for the Letby prosecution saw it differently. In a statement to police in 2021, seen by UnHerd, Professor Peter Hindmarsh of University College London, who was called as an expert witness by the prosecution over the cases of Baby F and Baby L, commented on a test on a blood sample taken from Baby Y on 3 November. It showed that while the boy’s insulin level was high, the concentration of C-peptide, a substance produced in tandem with insulin, in his blood was low. According to Hindmarsh, this meant Baby Y’s hypoglycemia must have been caused by “exogenous” insulin administered by someone in the neonatal unit.

Other prosecution experts agreed. Dewi Evans, the retired pediatrician who had played a central role in the police inquiry and also gave evidence in respect of most of the babies Letby was convicted of harming, also reviewed Baby Y’s medical records. He told police in a statement that he thought his first three seizures had been caused not by CHI, but the “trauma” of his difficult birth, agreeing with Hindmarsh that he had later been given “insulin from an external source”. A third prosecution expert, Dr Sandie Bohin, backed this position too.

However, we spoke to two practicing neonatal consultants who say all Baby Y’s “dusky episodes” and seizures could have been caused by naturally occurring hyperinsulinism, either transient or CHI — which, they say, is far more common than the prosecution experts appeared to believe. They say what matters most is that Baby Y had a confirmed diagnosis of CHI and still needed treatment long after leaving the Chester unit.

Shoo Lee’s expert panel has come to a similar conclusion. Its summary report, now being examined by the Criminal Cases Review Commission (CCRC), says naturally occurring hypoglycemia affects up to 40% of newborn babies, and that the relative levels of insulin and C-peptide seen in babies F, L and Y are also “not uncommon”. The timeline here is important too, because Baby Y’s first three seizures all took place before Letby ever encountered him.

If Soni’s statement had been given to Letby’s defense team, they might have called her as a witness — and explored why she thought Baby Y was suffering from CHI rather than a deliberate insulin overdose. That, in turn, might have enabled them to question the conclusions reached by the prosecution experts around babies F and L. In those cases, too, they claimed that the combination of high insulin and low C-peptide levels pointed to deliberate insulin poisoning.

We asked the Crown Prosecution Service why it failed to disclose Soni’s statement, but it refused to answer, stating only that Letby had been convicted and had lost her appeal. But its former chief, Lord Ken Macdonald KC, suggests the statement should likely have been shared. “The legal test is whether evidence is capable either of undermining the prosecution case or assisting the defense, and if it is, it should be disclosed before trial,” Macdonald says. “It sounds to me that the material you’ve found does meet that standard, and so should have been made available to Letby’s defense.”

Beyond the questions around Soni’s deleted statement, our investigation has uncovered a second key omission. Neither of the juries in Letby’s two trials was informed that a dangerous respiratory syncytial virus (RSV) outbreak was ravaging the neonatal ward. This breakout was so severe that it forced the unit’s closure to new admissions — on 18 February 2016, the day after Baby K, one of the babies Letby was convicted of trying to murder, was moved from Chester to another hospital, where she later died.

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This is despite the fact that staff at the hospital were clearly concerned about the outbreak. UnHerd has seen a police statement by Dr Bohin, dated 2021, which refers to a “management synopsis report” on the virus, a spreadsheet listing all the babies who contracted it, along with references to several similar documents about the outbreak.

Until the eve of her first trial, Letby was charged with murdering Baby K, but the prosecution asked the judge to alter the indictment to attempted murder. It was confident Letby had dislodged the child’s breathing tube — but, to quote the prosecution counsel, “we cannot and do not say” that what she did caused the girl’s death. He did not mention the virus sweeping through the neonatal unit when Baby K was born. For her part, Bohin made the dangers of RSV clear. “Its effects are usually mild, but in preterm infants RSV is responsible for significant mortality and morbidity,” she told police in a statement. “The virus is spread easily and can live for prolonged periods on hands and hard surfaces. There is no cure.” Baby K’s medical records do not say whether she was tested for RSV, but she had been in respiratory distress, and if she did contract the virus at Chester, it may have contributed to her death.

Meanwhile Shoo Lee’s expert panel is skeptical of the prosecution’s argument that Letby tried to kill Baby K by dislodging her breathing tube, suggesting instead that the hospital had used the wrong size tube, causing air to leak.

Beyond the medical questions here, these documents could have implications for Letby’s future. The CCRC and the Court of Appeal can consider fresh evidence that might, in theory, have been available at the time of a trial — if they are satisfied that to do so would be in the interests of justice. But they also have to consider whether there is a reasonable explanation why it was not used by the defense, and there is no stronger reason than a failure by the prosecution to disclose evidence to start with. This is ultimately why the cases of Baby Y and Baby K may yet be so significant.

But did the chart tell the whole story?

Today, I can reveal evidence that casts fresh doubt. Derived from unpublished notes taken by a police officer at the time, it shows that, when Evans first detailed his findings to senior detectives from Cheshire Police and National Crime Agency experts over two days in October 2017, a number of his conclusions were strikingly different from those presented in court. And yet, the jury was never told about them.

Some of the discrepancies were relatively minor. For example, during Letby’s trial, the court was told by one of her colleagues that the health of a victim known as “Baby K” began to deteriorate after Letby deliberately dislodged her breathing tube. Reviewing the evidence in 2017, however, Evans originally told the police he believed Baby K died from natural causes and had “no suspicions” about the case. She had been born prematurely and simply “deteriorated”. In an earlier email to police, he pointed out that the health of such infants was often “unstable”. Letby was convicted of attempting to murder Baby K after a retrial last year, after the first jury could not reach a verdict on her case.

Other discrepancies contained within the official notes, written by Detective Sergeant Jane Moore, are more serious. In fact, according to Evans’s initial analysis, and as the below chart illustrates, Letby was not in the hospital when 10 of the 28 incidents he described as “suspicious” took place — more than a third of them. In other words, if Evans’s initial analysis suggested there had been multiple murders, Letby could not have committed all of them.

The inconsistencies began with the very first case — that of “Baby A”, who died on 8 June 2015, supposedly from having air injected into his stomach. The jury convicted Letby of murdering him after hearing that, after being born nine weeks premature, his condition deteriorated after Letby came on shift at 7.30pm. According to Moore’s meeting notes, however, Evans regarded the entire period after 5pm as “relevant” — i.e. the baby could have started to deteriorate before Letby arrived.

Another case Evans thought “suspicious” was that of Baby A’s twin, “Baby B”. The trial was told she collapsed and almost died at about 12.30am on 10 June, when Letby was on duty, and the nurse was later convicted of attempted murder. But when Evans met the police in 2017, he suggested the child had been subjected to a further attack on 19 June, when at 10.50pm she was either injected with “intravenous air” or smothered by a “hand over [her] face”. Evans said the police should focus their inquiries on the period that started at 9.30pm. However, nothing was said about this second incident at Letby’s trial, probably because she clocked off that day at 8pm.

The next child Letby was convicted of murdering was “Baby C”. It is already known that Evans altered his account of his death at the trial. He had claimed in a report written a few weeks before the trial started that x-ray evidence demonstrated that Baby C had been fatally injured on 12 June, 2015, when air was injected into his stomach via a nasogastric tube.

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However, by the time Evans gave evidence, it had emerged that Letby had not been at work on 12 June — prompting him to change his story, and claim she must have administered the fatal injection the following day. Cross-examining Evans, Letby’s barrister, Ben Myers KC, accused him of massaging his evidence to avoid inconvenient truths. Evans said he “disagreed”, and it seems the jury believed him.

Yet concealed altogether from the trial was Evans’s account to police in October 2017. Then, Moore’s notes say, he thought the critical event was the discovery at 7am on 12 June that Baby C’s “UV line” — an umbilical catheter used to administer fluids and medicine to sick newborn babies — had “come out”. “Can this be explained?” Evans asked. “If the UV line being out is suspicious, then death is suspicious.” Again, there was no mention of the fact that Letby was not on duty on 12 June. In fact, she had not been at work at all since the baby’s birth on 10 June.

It’s a similar story with the case of “Baby I” who, according to Evans at the trial, Letby murdered by injecting air into her bloodstream on 22 October 2015. But in 2017, he told the police about an earlier “suspicious” incident, when Baby I went into respiratory arrest at 10pm on 30 September and began to struggle to breathe. Her abdomen was “distended”, Evans said in his initial report to police, suggesting she had been attacked with “air into stomach”, which might have been injected through her “milk line”. The jury did hear that her condition worsened that night, but no evidence that this was Letby’s fault. After all, her roster record makes clear she was not at work that night.

And then there’s the death of Baby O — the case that persuaded Evans that there was a murderer at the Countess of Chester within 10 minutes of starting to review the hospital’s medical records he was shown by police. At the trial, the jury was told Baby O had been “stable” until the afternoon of 23 June 2016, when he suffered a “remarkable deterioration”. But when Evans met the police in 2017, he said there were already “problems before collapse”, and that by 5am, his heart rate was “climbing” — a sure sign he was not in good health. This was, he concluded, “suspicious”, and what had happened “overnight 22–23 [June]” was “relevant”.

He may well have been correct. But on the night in question, Letby was not at work. Indeed, last month, MP David Davis revealed in Parliament that an assessment of Baby O’s medical notes by two eminent neonatal consultants had found that the “impact injury” to her liver that triggered severe internal bleeding was inflicted not by Letby but by a consultant pediatrician, who inadvertently stabbed the organ with a needle. The same doctor, Davis said, went on to become one of Letby’s principal accusers, and gave evidence at her trial.

Suffice it to say that none of this featured during Letby’s trial — though other doubts about Evans did surface. It emerged, for instance, that the Court of Appeal’s Lord Justice Jackson had taken the extraordinary step of writing to the trial judge, Mr Justice Goss, detailing how a report drawn up by Evans in an unrelated family case had been dismissed as “worthless”. Evans, he claimed, had breached his duty as an expert by deciding on the outcome he wanted, then “working out an explanation” to achieve it. “Of greatest concern”, Jackson wrote, “Dr Evans makes no effort to provide a balanced opinion,” suggesting this might amount to “a breach of proper professional conduct”.

This warning was not taken lightly. I have seen a further unpublished document that suggests that Jackson’s email to the judge produced alarm in the prosecution camp. It shows that on 9 January 2023, Crown Prosecution Service lawyers held a meeting with Evans in which they posed scripted questions about Jackson’s criticisms. He appeared untroubled, saying: “I stand by my report.”

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Next week, a panel of international experts is set to offer further challenges to the prosecution case, and is expected to produce a report based on close analysis of the babies’ records, saying Letby’s “victims” were not deliberately harmed or murdered. Meanwhile, the Court of Appeal has refused her permission to appeal her convictions and found that there could have been “no arguable basis” for excluding the evidence given by Evans from the trial.

As for Evans, in October last year, he submitted yet another report on the babies’ deaths to Cheshire Police — which has, thus far, refused to disclose it to Letby’s new lawyer, Mark McDonald, who is fighting to get the case reopened. Beyond this, Evans is reluctant to be drawn on his initial report. When I put detailed, written questions about the issues raised in this article to Evans, he declined to respond, saying only: “I decided, as of mid-December last year, to make no comment pending the completion of Lady Thirlwall’s [public] Inquiry (summing up in March I understand) and Cheshire Police completing their investigation.”

I also put detailed questions to the Cheshire police but they also declined to answer them. Their spokesperson said the force was still investigating deaths at the Countess of Chester Hospital and at a hospital in Liverpool, adding: “Cheshire Constabulary has declined to be involved in much of the ongoing commentary within the media… There is a significant public interest in the reporting of these matters, however, every story that is published, statement made, or comment posted online that refers to the specific details of a live investigation can impede the course of justice and cause further distress to the families concerned. It is these families and the ongoing investigations that remain our primary focus.”

The late Appeal Court judge Lord Denning once commented that if it were true that the Birmingham Six, the men convicted of killing 21 people by bombing pubs in 1974, were innocent, this would constitute a “an appalling vista” that he found impossible to contemplate. In 1991, fresh evidence made it clear that they were victims of one of the worst miscarriages of justice in English legal history, and they all walked free.

In the wake of her convictions, Letby, like the Six, was portrayed as an evil monster, a woman who fully deserved to rot in prison for the rest of her life. What an appalling vista if she were revealed to be a victim, not a perpetrator.

The defendant, picked up in a routine traffic stop, denied being the man who’d carried out an inside-job fraud at the shop where he’d worked some years earlier. His name, ID and credit cards matched, and apart from a little weight gain and a new hairdo, he looked exactly like the two available photos of the criminal. A defence “facial mapping” expert pointed out several differences, but our own expert explained them away and supported the identification. Forceful cross-examination of the defence expert led him to concede that he might be wrong. But a last-minute double-check by the police unearthed — finally — one of the fraudster’s former colleagues. His probity beyond question, he was brought to court to say whether he recognised the man in the dock. A guileless “no”, followed by an unfakeably perplexed chuckle when it was suggested it might be the man he used to work with, confirmed the defendant’s innocence. (The two might well have been cousins sharing an identity, but that’s another story.)

It immediately became blindingly obvious to me that the photos of the two men were not the same person at all. You could tell at a glance! But when you are given a theory, and you run with it, it takes something big to bring about that paradigm shift.

In the case of the nurse Lucy Letby, a theory that she was a murderer was alighted upon by one of the ward’s consultant paediatricians very early on — after only a very few of the unexplained deaths or collapses that ended up on her indictment — and the theory was based almost entirely on her having been on shift at the relevant times. There can be little doubt that there was not, at that stage, enough evidence for murder to be anything other than a remote possibility. So either it was a very lucky guess, or this theory was later taken up by others in a way that led to inaccurate expert medical opinion, the misleading presentation of evidence, and a terrible miscarriage of justice.

Following her conviction in August 2023, for murdering seven babies and trying to kill six others, a 13,000-word piece in The New Yorker told a compelling story of her likely innocence. More recently, and once the reporting restrictions were lifted after her conviction in early July in a retrial for the attempted murder of one of the babies on which the jury could not agree, long pro-defence pieces have appeared in The Guardian, Telegraph, Independent and Private Eye. Also recently published — withheld pending the outcome of the retrial — is the written judgement of the Court of Appeal, who refused her application for leave to appeal.

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An appeal first goes to a single judge, who makes a decision on the papers whether to let you argue it at a hearing. In Letby’s case, that judge was Sir Robin Spencer, who turned her application down. Coincidentally — or perhaps not — Spencer was the barrister who led the prosecution of Sally Clark in 1999, who was wrongly convicted of killing two of her young children. That notorious miscarriage of justice, as well as others from the same era, such as Angela Cannings and Donna Anthony, was based on flawed statistical evidence about cot death from the since-discredited and disgraced paediatrician Sir Roy Meadow.

“Unknown unknowns” back then included the possibility of a genetic factor that could explain multiple cot deaths in the same family. It was nearly 20 years before a good candidate was discovered. A growing collection of lawyers, doctors and statisticians believe that something similar — flawed statistics and paediatric misdiagnosis without due respect for unknown unknowns — may have occurred in Letby’s case.

If the first Court of Appeal judge turns you down, you can try again at an oral hearing in front of three others. Letby was again unsuccessful. Their written judgement is long, detailed and illuminating. But it will not, I suspect, relieve much anxiety about a possible miscarriage of justice.

The main issue in the original trial, and indeed the retrial, was whether the deaths and other “events” were deliberately caused. Letby’s biggest problem here was the two allegations of attempted murder by insulin — Babies F and L, committed some eight months apart. Blood test results showed high insulin accompanied by low levels of a chemical called c-peptide. When the pancreas produces insulin, it produces similar levels of c-peptide. Therefore, the Crown said, the insulin was not produced in the body. It was, in other words, artificial. Therefore, it was an attempt to kill.

The inference to foul play with these two babies is arguably much stronger than it is with the more numerous diagnoses of air embolism, which was a more subjective opinion, more dependent on the statistically questionable “shift-pattern” evidence. The two guilty verdicts on the insulin counts were the first the jury returned — the next not coming for three days — and were two of only three unanimous convictions in the trial.

The Court of Appeal records that “the integrity of the blood samples and reliability of the biochemical testing was challenged by [defence KC] Mr Myers. However, in her evidence at trial, the applicant admitted that both babies had been poisoned by insulin, but denied that she was the poisoner.” The Crown made much of this “admission”, but the point is seriously unfair.

Letby was being pushed quite hard to swallow an inference which she was in no position to accept or deny. Her evidence was that she did not poison them — simple as that. So her opinion of the validity of the blood samples and the inferences to be drawn from them is neither here nor there.

This is a common practice by prosecutors, easy to lapse into in the heat of the moment. I’ve done it myself. “So it’s all just a big coincidence is it? Or have these two complainants put their heads together?” — that sort of thing. You’re trying to make a little speech and put the defendant on the back foot, in the course of questioning them about matters within their knowledge — which is what you should confine yourself to. But a defendant’s answers to such questions are not a sound basis for treating a prosecution inference as agreed by the defence. If the reliability of a blood sample is not agreed, or an inference not accepted, there the matter should rest. Her barrister explicitly did not concede the accuracy of the results in his speech to the jury, much less the inferences to be drawn.

And it seems there may be good reason not to accept their validity. The Guardian has reported serious concerns about the results from senior experts in the field. The Telegraph quotes guidance from the relevant lab saying that its tests are “not suitable” where the introduction of artificial insulin is suspected. And The New Yorker piece quotes the Crown’s expert as saying that there was a third insulin case, with similar blood results, not attributed to Letby. The samples cannot be tested again as they were destroyed soon after they were taken — because the babies’ blood sugar levels recovered and there was no suspicion of wrongdoing at the time.

But even without these post-conviction queries, if this were the only evidence, and these the only allegations, I would hope no jury would convict a nurse for attempted murder on that basis. The timing and detail of the insulin results meant that the Crown had to say that not only must Letby have poisoned the feeding bags she herself put up, but also must have poisoned one or more other bags in the store cupboard so that they would unwittingly be administered by other nurses after she had left. (The bags themselves, of course, are long gone.) Would you feel sure a random nurse tried to kill two babies eight months apart, or would you think this might be an instance of why the lab says its tests are “not suitable” for proving the presence of artificial insulin? But of course, this was not the only evidence against her.

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There are also Letby’s online searches for bereaved families, and the collection of medical notes found at her home relating to some of the events she was charged with. Neither of these, I feel, are anything like as bad as they look: they both stand against a background of a much larger number of non-suspicious searches and medical notes.

But for many observers, and perhaps for the jury, the “confession” note is persuasive. For criminal lawyers, less so. That’s because false confessions are surprisingly common. So much so, that a law was enacted to prohibit a confession being given in evidence unless the prosecution can prove beyond reasonable doubt that it was not obtained by oppression or as a result of anything “said or done” that was likely to render a confession unreliable.

How could any nurse subjected to a long investigation into whether they have been murdering babies find the process anything but oppressive? If the case against her can be proved beyond reasonable doubt, one might think it could — and even should — be proved without the mixed inculpatory-exculpatory scrawls of a woman on the brink.

Almost all of the rest of the convictions — that is, all seven murders and three of the alleged attempts — involved the deliberate injection of air, leading to air embolism. One ground of Letby’s appeal impugned the prosecution experts’ diagnosis-by-exclusion — i.e. “we don’t know what else it could be, so it is likely to be air embolism” — as well as their diagnosis on the basis of various kinds of unusual skin discolouration that were observed. The 1989 research paper relied on by the Crown’s experts, Pulmonary Vascular Air Embolism in the Newborn, is very short, and says relatively little about skin discolouration other than that, in about 10% of the 50 or so cases they reviewed, “migrating pallor” in the skin was observed — which, it was not disputed, may be caused by other things.

The Court of Appeal was not impressed by this complaint, or by fresh evidence from the surviving author of the 1989 paper in which he criticised the use to which his research had been put in the Letby case. The experts were entitled to use skin discolouration as supportive of the diagnosis, the court said, as being “consistent with” embolism; the fresh evidence did not undermine that; and the lack of any other plausible explanation is significant.

“Consistent with” does a lot of heavy lifting for the Crown in criminal trials. A piece of evidence only helps to prove something if — and to the extent that — there is a difference between the probability of finding the evidence if the hypothesis (e.g. guilt, or air embolism) is true, and the probability of finding it if it is false. More precisely, it is the ratio between these two that determines the strength of the evidence in either direction. That, in a nutshell, is Bayes’s theorem. So, if you can say no more than that a piece of evidence is “consistent with” the hypothesis, but it is also “consistent with” its negation, and you can’t say how much more “consistent” it is with one rather than the other, then the evidence does not assist you either way. For example, if a prosecutor says “the fact you were wearing gloves makes you more likely to be guilty of this robbery”, that evidence is in fact worthless if it’s winter time and everyone is wearing gloves: wearing gloves is “consistent with” both guilt and innocence, to the same degree.

It seems, though, that the experts used skin discolouration as more than merely “consistent with” air embolism. They used it to prove, to some extent, that diagnosis. Which they are entitled to do, if it is a justifiable professional opinion, but it does point to a fundamental danger with expert evidence: the difficulty a jury has in getting behind the curtain and judging what weight to give it.

One way of achieving this is to look at whether the experts have been wrong before. In the case of the lead expert in Letby’s case, Dr Evans, the defence discovered halfway through the trial that he had been utterly excoriated in a ruling by a Court of Appeal judge in a different case. “No effort to provide a balanced opinion… No attempt made to engage with the powerful contradictory indicators… The report has the hallmarks of an exercise ‘working out an explanation’ that exculpates the applicants… tendentious and partisan expressions of opinion that are outside [his] professional competence.” That level of criticism from a senior judge is not made lightly: they know it can be career-ending. And if the Crown had known of it in advance, I suspect they would have looked for a different expert.

Now, this was on one view a gift to the defence: they could now undermine Dr Evans’s credibility by cross-examining him on it in front of the jury. True, his conclusions in the Letby trial were supported by other experts who had checked his work, but the defence argued that those experts were merely rubber-stamping his opinions. The jury may have had a difficult task in assessing what weight to give the criticisms of Evans’s previous work — one with somewhat binary options: disregard his evidence, or believe it. Ultimately, the Court of Appeal upheld the trial judge’s rejection of the defence’s application to exclude his evidence. It might not have been an easy decision for the trial judge, or the Court of Appeal, and although it may have been the correct decision, I cannot imagine any lawyer would claim the situation overall was entirely satisfactory.

Zooming out, the big question remains: without knowing of the police investigation and the suspicion of foul-play, and the fact that one nurse was on duty for many or all of the incidents under review, would any expert have diagnosed air embolism by the deliberate injection of air, in any of those cases? Certainly none of the pathologists did at the time.

In his recent Private Eye piece, Dr Phil Hammond gives a startling statistic: in a review of more than 1,000 cases of infant death in south-east London, “the cause of mortality was unexplained for about half of the newborns who had died unexpectedly, even after post-mortem examinations”.

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So, the shift-pattern evidence — i.e. the fact that Letby was on duty for so many unexplained deaths and collapses — does seem to have been central to the case against her. And statisticians are questioning its probative power. Even leaving aside the issue of deaths and other negative events that occurred when Letby was not on duty, coincidences of that scale occur — just as someone will win the lottery each week, at odds of 14 million to one.

Also very unusual is the fact that Dr Evans “pitched” his services to the police, when he heard about the investigation in the media. This is not the way experts are normally selected. The defence argued it indicated pro-prosecution bias. The Court of Appeal disagreed.

And then there’s the fact that in the last few weeks Dr Evans has commented publicly on the defence not calling their own expert, who was advising them throughout the trial. Indeed, that expert, Dr Hall, has himself gone on record saying that he “does not know” why he was not called. And the same question was posed by the Court of Appeal, somewhat pointedly: without having called their own expert, can the defence really complain if the jury agree with the Crown’s?

It’s an entirely valid question. But I would be slow to assume the worst. One reason the defence might have chosen not to call Dr Hall (and I am speculating here) is if they felt they had so thoroughly undermined Dr Evans that they were better off leaving it at that. That would be a big tactical call, but covering your own back is not always consistent with protecting your client’s interests.

So, for example: if your own expert would contradict much, but not all, of the Crown’s expert’s conclusions, and you believe the credibility of the Crown’s expert has been totally destroyed, you might judge it prudent not to put your own expert in the witness box. Avoiding the potential for future criticism and hindsight regret by putting your client in what you believe will be a worse position — by calling your own expert — is not a boss move. This is all speculation, but I guarantee that careful thought went into the decision not to call Dr Hall. None of this, though, would mean the conviction is safe.

There is evidence against Letby. And the fact that it’s “circumstantial” is not a valid criticism: circumstantial evidence can be, and often is, utterly overwhelming. But this is where comparison with the Sally Clark case really comes in. The famous statistical error in that trial was that the expert Professor Roy Meadow wrongly treated two putative cot deaths as probabilistically independent. But arguably the more important point is that while the probability of “coincidence” might indeed have been very low, so too is the probability of a mother murdering her children. In Letby’s case, the chance that a randomly selected nurse is a baby killer is vanishingly small. As Carl Sagan put it, paraphrasing the French mathematician Laplace, extraordinary claims require extraordinary evidence.

Defence lawyers often say to juries that their clients start off innocent: the fact that they are in the dock is no evidence at all. But this is not quite right. If the probability of guilt is zero, no amount of evidence — that is, no Bayesian likelihood ratios, no matter how high or how numerous — will alter that chance. Rather, a defendant’s prior probability of guilt — that is, before any evidence has been called — is just very, very small. But not all crimes are equally common. A randomly selected member of the public is much more likely to have stolen a bike — given the number of bikes that get stolen — than a randomly selected nurse is to be a serial killer of her patients. But there isn’t much room for these fundamental concepts to be explored and explained to a jury.

There has been some commentary to the effect that those protesting Letby’s innocence should pipe down, that they are behaving irresponsibly. But miscarriages of justice do occur, and the Court of Appeal sometimes gets things wrong.

The only route for Letby now would be a referral back to the Court of Appeal by the Criminal Cases Review Commission, which would require substantial new evidence of one sort or another. The process is not quick, and the Commission do not refer many cases. But I would not be surprised if one day it happens. I hope it does. If Letby has been wrongly convicted, it would be one of the worst miscarriages of justice we have ever seen. If she is guilty, well, true verdicts, like true scientific theories, are only strengthened by rigorous testing and challenge.

***

This article was first published on 24 July 2024.

The defendant, picked up in a routine traffic stop, denied being the man who’d carried out an inside-job fraud at the shop where he’d worked some years earlier. His name, ID and credit cards matched, and apart from a little weight gain and a new hairdo, he looked exactly like the two available photos of the criminal. A defence “facial mapping” expert pointed out several differences, but our own expert explained them away and supported the identification. Forceful cross-examination of the defence expert led him to concede that he might be wrong. But a last-minute double-check by the police unearthed — finally — one of the fraudster’s former colleagues. His probity beyond question, he was brought to court to say whether he recognised the man in the dock. A guileless “no”, followed by an unfakeably perplexed chuckle when it was suggested it might be the man he used to work with, confirmed the defendant’s innocence. (The two might well have been cousins sharing an identity, but that’s another story.)

It immediately became blindingly obvious to me that the photos of the two men were not the same person at all. You could tell at a glance! But when you are given a theory, and you run with it, it takes something big to bring about that paradigm shift.

In the case of the nurse Lucy Letby, a theory that she was a murderer was alighted upon by one of the ward’s consultant paediatricians very early on — after only a very few of the unexplained deaths or collapses that ended up on her indictment — and the theory was based almost entirely on her having been on shift at the relevant times. There can be little doubt that there was not, at that stage, enough evidence for murder to be anything other than a remote possibility. So either it was a very lucky guess, or this theory was later taken up by others in a way that led to inaccurate expert medical opinion, the misleading presentation of evidence, and a terrible miscarriage of justice.

Following her conviction last August, for murdering seven babies and trying to kill six others, a 13,000-word piece in The New Yorker told a compelling story of her likely innocence. More recently, and once the reporting restrictions were lifted after her conviction three weeks ago in a retrial for the attempted murder of one of the babies on which the jury could not agree, long pro-defence pieces have appeared in The Guardian, Telegraph, Independent and Private Eye. Also recently published — withheld pending the outcome of the retrial — is the written judgement of the Court of Appeal, who refused her application for leave to appeal.

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An appeal first goes to a single judge, who makes a decision on the papers whether to let you argue it at a hearing. In Letby’s case, that judge was Sir Robin Spencer, who turned her application down. Coincidentally — or perhaps not — Spencer was the barrister who led the prosecution of Sally Clark in 1999, who was wrongly convicted of killing two of her young children. That notorious miscarriage of justice, as well as others from the same era, such as Angela Cannings and Donna Anthony, was based on flawed statistical evidence about cot death from the since-discredited and disgraced paediatrician Sir Roy Meadow.

“Unknown unknowns” back then included the possibility of a genetic factor that could explain multiple cot deaths in the same family. It was nearly 20 years before a good candidate was discovered. A growing collection of lawyers, doctors and statisticians believe that something similar — flawed statistics and paediatric misdiagnosis without due respect for unknown unknowns — may have occurred in Letby’s case.

If the first Court of Appeal judge turns you down, you can try again at an oral hearing in front of three others. Letby was again unsuccessful. Their written judgement is long, detailed and illuminating. But it will not, I suspect, relieve much anxiety about a possible miscarriage of justice.

The main issue in the original trial, and indeed the retrial, was whether the deaths and other “events” were deliberately caused. Letby’s biggest problem here was the two allegations of attempted murder by insulin — Babies F and L, committed some eight months apart. Blood test results showed high insulin accompanied by low levels of a chemical called c-peptide. When the pancreas produces insulin, it produces similar levels of c-peptide. Therefore, the Crown said, the insulin was not produced in the body. It was, in other words, artificial. Therefore, it was an attempt to kill.

The inference to foul play with these two babies is arguably much stronger than it is with the more numerous diagnoses of air embolism, which was a more subjective opinion, more dependent on the statistically questionable “shift-pattern” evidence. The two guilty verdicts on the insulin counts were the first the jury returned — the next not coming for three days — and were two of only three unanimous convictions in the trial.

The Court of Appeal records that “the integrity of the blood samples and reliability of the biochemical testing was challenged by [defence KC] Mr Myers. However, in her evidence at trial, the applicant admitted that both babies had been poisoned by insulin, but denied that she was the poisoner.” The Crown made much of this “admission”, but the point is seriously unfair.

Letby was being pushed quite hard to swallow an inference which she was in no position to accept or deny. Her evidence was that she did not poison them — simple as that. So her opinion of the validity of the blood samples and the inferences to be drawn from them is neither here nor there.

This is a common practice by prosecutors, easy to lapse into in the heat of the moment. I’ve done it myself. “So it’s all just a big coincidence is it? Or have these two complainants put their heads together?” — that sort of thing. You’re trying to make a little speech and put the defendant on the back foot, in the course of questioning them about matters within their knowledge — which is what you should confine yourself to. But a defendant’s answers to such questions are not a sound basis for treating a prosecution inference as agreed by the defence. If the reliability of a blood sample is not agreed, or an inference not accepted, there the matter should rest. Her barrister explicitly did not concede the accuracy of the results in his speech to the jury, much less the inferences to be drawn.

And it seems there may be good reason not to accept their validity. The Guardian has reported serious concerns about the results from senior experts in the field. The Telegraph quotes guidance from the relevant lab saying that its tests are “not suitable” where the introduction of artificial insulin is suspected. And The New Yorker piece quotes the Crown’s expert as saying that there was a third insulin case, with similar blood results, not attributed to Letby. The samples cannot be tested again as they were destroyed soon after they were taken — because the babies’ blood sugar levels recovered and there was no suspicion of wrongdoing at the time.

But even without these post-conviction queries, if this were the only evidence, and these the only allegations, I would hope no jury would convict a nurse for attempted murder on that basis. The timing and detail of the insulin results meant that the Crown had to say that not only must Letby have poisoned the feeding bags she herself put up, but also must have poisoned one or more other bags in the store cupboard so that they would unwittingly be administered by other nurses after she had left. (The bags themselves, of course, are long gone.) Would you feel sure a random nurse tried to kill two babies eight months apart, or would you think this might be an instance of why the lab says its tests are “not suitable” for proving the presence of artificial insulin? But of course, this was not the only evidence against her.

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There are also Letby’s online searches for bereaved families, and the collection of medical notes found at her home relating to some of the events she was charged with. Neither of these, I feel, are anything like as bad as they look: they both stand against a background of a much larger number of non-suspicious searches and medical notes.

But for many observers, and perhaps for the jury, the “confession” note is persuasive. For criminal lawyers, less so. That’s because false confessions are surprisingly common. So much so, that a law was enacted to prohibit a confession being given in evidence unless the prosecution can prove beyond reasonable doubt that it was not obtained by oppression or as a result of anything “said or done” that was likely to render a confession unreliable.

How could any nurse subjected to a long investigation into whether they have been murdering babies find the process anything but oppressive? If the case against her can be proved beyond reasonable doubt, one might think it could — and even should — be proved without the mixed inculpatory-exculpatory scrawls of a woman on the brink.

Almost all of the rest of the convictions — that is, all seven murders and three of the alleged attempts — involved the deliberate injection of air, leading to air embolism. One ground of Letby’s appeal impugned the prosecution experts’ diagnosis-by-exclusion — i.e. “we don’t know what else it could be, so it is likely to be air embolism” — as well as their diagnosis on the basis of various kinds of unusual skin discolouration that were observed. The 1989 research paper relied on by the Crown’s experts, Pulmonary Vascular Air Embolism in the Newborn, is very short, and says relatively little about skin discolouration other than that, in about 10% of the 50 or so cases they reviewed, “migrating pallor” in the skin was observed — which, it was not disputed, may be caused by other things.

The Court of Appeal was not impressed by this complaint, or by fresh evidence from the surviving author of the 1989 paper in which he criticised the use to which his research had been put in the Letby case. The experts were entitled to use skin discolouration as supportive of the diagnosis, the court said, as being “consistent with” embolism; the fresh evidence did not undermine that; and the lack of any other plausible explanation is significant.

“Consistent with” does a lot of heavy lifting for the Crown in criminal trials. A piece of evidence only helps to prove something if — and to the extent that — there is a difference between the probability of finding the evidence if the hypothesis (e.g. guilt, or air embolism) is true, and the probability of finding it if it is false. More precisely, it is the ratio between these two that determines the strength of the evidence in either direction. That, in a nutshell, is Bayes’s theorem. So, if you can say no more than that a piece of evidence is “consistent with” the hypothesis, but it is also “consistent with” its negation, and you can’t say how much more “consistent” it is with one rather than the other, then the evidence does not assist you either way. For example, if a prosecutor says “the fact you were wearing gloves makes you more likely to be guilty of this robbery”, that evidence is in fact worthless if it’s winter time and everyone is wearing gloves: wearing gloves is “consistent with” both guilt and innocence, to the same degree.

It seems, though, that the experts used skin discolouration as more than merely “consistent with” air embolism. They used it to prove, to some extent, that diagnosis. Which they are entitled to do, if it is a justifiable professional opinion, but it does point to a fundamental danger with expert evidence: the difficulty a jury has in getting behind the curtain and judging what weight to give it.

One way of achieving this is to look at whether the experts have been wrong before. In the case of the lead expert in Letby’s case, Dr Evans, the defence discovered halfway through the trial that he had been utterly excoriated in a ruling by a Court of Appeal judge in a different case. “No effort to provide a balanced opinion… No attempt made to engage with the powerful contradictory indicators… The report has the hallmarks of an exercise ‘working out an explanation’ that exculpates the applicants… tendentious and partisan expressions of opinion that are outside [his] professional competence.” That level of criticism from a senior judge is not made lightly: they know it can be career-ending. And if the Crown had known of it in advance, I suspect they would have looked for a different expert.

Now, this was on one view a gift to the defence: they could now undermine Dr Evans’s credibility by cross-examining him on it in front of the jury. True, his conclusions in the Letby trial were supported by other experts who had checked his work, but the defence argued that those experts were merely rubber-stamping his opinions. The jury may have had a difficult task in assessing what weight to give the criticisms of Evans’s previous work — one with somewhat binary options: disregard his evidence, or believe it. Ultimately, the Court of Appeal upheld the trial judge’s rejection of the defence’s application to exclude his evidence. It might not have been an easy decision for the trial judge, or the Court of Appeal, and although it may have been the correct decision, I cannot imagine any lawyer would claim the situation overall was entirely satisfactory.

Zooming out, the big question remains: without knowing of the police investigation and the suspicion of foul-play, and the fact that one nurse was on duty for many or all of the incidents under review, would any expert have diagnosed air embolism by the deliberate injection of air, in any of those cases? Certainly none of the pathologists did at the time.

In his recent Private Eye piece, Dr Phil Hammond gives a startling statistic: in a review of more than 1,000 cases of infant death in south-east London, “the cause of mortality was unexplained for about half of the newborns who had died unexpectedly, even after post-mortem examinations”.

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So, the shift-pattern evidence — i.e. the fact that Letby was on duty for so many unexplained deaths and collapses — does seem to have been central to the case against her. And statisticians are questioning its probative power. Even leaving aside the issue of deaths and other negative events that occurred when Letby was not on duty, coincidences of that scale occur — just as someone will win the lottery each week, at odds of 14 million to one.

Also very unusual is the fact that Dr Evans “pitched” his services to the police, when he heard about the investigation in the media. This is not the way experts are normally selected. The defence argued it indicated pro-prosecution bias. The Court of Appeal disagreed.

And then there’s the fact that in the last few weeks Dr Evans has commented publicly on the defence not calling their own expert, who was advising them throughout the trial. Indeed, that expert, Dr Hall, has himself gone on record saying that he “does not know” why he was not called. And the same question was posed by the Court of Appeal, somewhat pointedly: without having called their own expert, can the defence really complain if the jury agree with the Crown’s?

It’s an entirely valid question. But I would be slow to assume the worst. One reason the defence might have chosen not to call Dr Hall (and I am speculating here) is if they felt they had so thoroughly undermined Dr Evans that they were better off leaving it at that. That would be a big tactical call, but covering your own back is not always consistent with protecting your client’s interests.

So, for example: if your own expert would contradict much, but not all, of the Crown’s expert’s conclusions, and you believe the credibility of the Crown’s expert has been totally destroyed, you might judge it prudent not to put your own expert in the witness box. Avoiding the potential for future criticism and hindsight regret by putting your client in what you believe will be a worse position — by calling your own expert — is not a boss move. This is all speculation, but I guarantee that careful thought went into the decision not to call Dr Hall. None of this, though, would mean the conviction is safe.

There is evidence against Letby. And the fact that it’s “circumstantial” is not a valid criticism: circumstantial evidence can be, and often is, utterly overwhelming. But this is where comparison with the Sally Clark case really comes in. The famous statistical error in that trial was that the expert Professor Roy Meadow wrongly treated two putative cot deaths as probabilistically independent. But arguably the more important point is that while the probability of “coincidence” might indeed have been very low, so too is the probability of a mother murdering her children. In Letby’s case, the chance that a randomly selected nurse is a baby killer is vanishingly small. As Carl Sagan put it, paraphrasing the French mathematician Laplace, extraordinary claims require extraordinary evidence.

Defence lawyers often say to juries that their clients start off innocent: the fact that they are in the dock is no evidence at all. But this is not quite right. If the probability of guilt is zero, no amount of evidence — that is, no Bayesian likelihood ratios, no matter how high or how numerous — will alter that chance. Rather, a defendant’s prior probability of guilt — that is, before any evidence has been called — is just very, very small. But not all crimes are equally common. A randomly selected member of the public is much more likely to have stolen a bike — given the number of bikes that get stolen — than a randomly selected nurse is to be a serial killer of her patients. But there isn’t much room for these fundamental concepts to be explored and explained to a jury.

There has been some commentary to the effect that those protesting Letby’s innocence should pipe down, that they are behaving irresponsibly. But miscarriages of justice do occur, and the Court of Appeal sometimes gets things wrong.

The only route for Letby now would be a referral back to the Court of Appeal by the Criminal Cases Review Commission, which would require substantial new evidence of one sort or another. The process is not quick, and the Commission do not refer many cases. But I would not be surprised if one day it happens. I hope it does. If Letby has been wrongly convicted, it would be one of the worst miscarriages of justice we have ever seen. If she is guilty, well, true verdicts, like true scientific theories, are only strengthened by rigorous testing and challenge.

For others — including me — the picture can flicker in and out. The Letby case is compelling because it invites interpretation. In a media where true crime reigns, the detailed session-by-session examination of evidence allowed anyone following to play not just jury but detective, weighing the subtleties of an autopsy or the reliability of a witness. This should, in theory, generate trust in the process; instead, it generates as many different processes as there are followers of the case, a million courtrooms in a million different heads all weighting the evidence in their way — and feeling convinced of their own conclusions.

From a certain distance, her guilt looks certain: the chart presented by the prosecution showing she was on the ward for all the suspicious deaths, the scrawled notes-to-self included the phrase “I AM EVIL I DID THIS”, the handover sheets she hoarded at home, the seemingly compulsive googling of the bereaved families months or years after their babies’ deaths.

Take a step back, and it threatens to collapse into dots. It’s this that feeds the population of Letby “truthers” who are gradually gathering strength. What if the chart is a case of “Texas sharpshooter fallacy”, when you draw a target to match a random selection of data? After all, babies on neonatal wards are by-definition fragile: sometimes they die, and they will die on someone’s shift, and chance might happen to put one person on all those shifts. Initially, the deaths Letby was convicted of were seen as unexplained, rather than violent. Then, once her colleagues had become suspicious, deaths when Letby was present became suspicious by definition.

The notes might not be evidence of a guilty conscience, but of a young woman under terrible pressure from false accusations sliding into a breakdown. The googling, a sign of her grief for the babies she couldn’t save. Of the hoarded documents, the defence pointed out that fewer than 10% of the papers found at her home referred to babies on the indictment, making them unlikely to be “trophies” — however strange it was that she had kept them at all.

The case against Letby was circumstantial and rested on four things: statistics, eyewitness testimony, forensics and the confession in her notes. These also happen to be among the forms of evidence most implicated in miscarriages of justice. Statistics can easily be misrepresented, as in the case of Sally Clark, wrongly convicted of killing her two babies after the paediatrician Professor Sir Roy Meadows gave expert evidence that failed to account for the possibility of a common congenital defect.

Eyewitness recall is compelling to juries, but vulnerable to human error. When Letby’s colleagues remembered her eagerness to be present during medical crises, or their own discomfort at her reluctance to take time off — was that the thought they had at the time, or a thought they had later, overlaid on the original impression? Confessions, similarly, feel decisive, but people can accuse themselves for many reasons unrelated to the facts of the case: from a desire for attention, from a need to feel important or because they are browbeaten into it.

Forensic evidence, though it has the surety of flesh, is interpretative. Letby’s defence did not introduce their own expert witnesses to challenge the prosecution’s but since the trial, others have raised doubts about the theory that Letby attacked babies using deliberate air embolisms and insulin poisoning, including in this week’s Private Eye by the medical columnist MD. (And if we’re talking about unlikely coincidences, Private Eye and Nadine Dorries coming to similar conclusions certainly counts.)

The Countess of Cheshire Hospital, where Letby worked, was overstretched and sometimes unsanitary. Her removal from the ward coincided with a collapse in mortality, but it also coincided with the unit being downgraded so it no longer treated babies in need of intensive care, so in reality this signifies almost nothing. (It’s worth noting, though, that even if the embolisms are discounted and the insulin poisoning deemed tendentious, there is still evidence of severe trauma and overfeeding in some of the babies.)

For those querying Letby’s guilt, the implication is that she has been made the scapegoat for systemic failures. Or as Nadine Dorries asked in the Daily Mail: “Has the British justice system thrown a young woman into jail for life in order to save the tarnished reputation of the National Health Service?” The lengthy New Yorker piece (still not available in the UK because of reporting restrictions) suggested that Letby’s defence was compromised by an unwillingness to criticise the “national religion” of the NHS.

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For the truthers, this has a satisfying flavour of cover-up — and would make Letby the main character in a Seventies-style conspiracy thriller, and them the brave truth tellers shaking down the system. Then again, her guilt also fits a narrative of institutional culpability. NHS managers ignored the warnings of their staff and even forced doctors to apologise to Letby for raising their concerns. Depending on how you fix your focus, the dots cohere and dissolve into different patterns.

Letby doesn’t quite fit any preconceived templates. Is she a poster girl for “privilege”? Novelist Joanne Harris tweeted that “The Letby case should teach us this: Too many people think ‘innocence’ looks white, middle-class, traditional, vulnerable, tearful. People who present this way are often unquestioningly believed and supported.” One weird response to this posture is that you can find the Letby-sympathetic dismissing the idea that she’s in anyway conventionally attractive (“she’s average at best” said one).

Letby wasn’t unquestioningly believed, clearly: her colleagues eventually thought the most unthinkable thing about her, and pursued their fears all the way to prosecution. Nor, however, does she match the pattern of the vulnerable young woman bullied by senior men that she would need to be for the more lurid truther versions of a plot against her to hold up: she stuck up for herself and won the backing of management.

What’s missing from the Letby case — and what the truthers want — is, crassly, a good story. We don’t know why she did it. Establishing motive is useful but not necessary for proving guilt, and the prosecution only hinted at possible explanations. Maybe she loved the drama of a death. Maybe she was seeking the attention of a married doctor she was said to be flirting with. Maybe she had munchausen’s by proxy. But even the police who led the investigation confess themselves baffled at her reasons.

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All that’s left is the oddness of a killer who put “x” and “lol” in her texts, and drank white wine on girls’ nights out, and supposedly recorded her kills in a twee diary with a picture of a knitted teddy on the cover. One of her friends called her “hun” in a consoling text message — “Oh hun, you need a break” — and that’s what Letby seems to be: a hun. A bit basic. This is not how serial killers are supposed to be in the popular imagination. They are meant to have either the terrifying self-control of Hannibal Lecter, or the chaotic impulsiveness of Leatherface in Texas Chainsaw Massacre. They are meant to get something from their violence.

Justice is not only about the facts. It is also about turning those facts into an arc that makes sense of the senseless. This is not extraneous to its core function, but fundamental to the way it allows society to move on from shocking acts of brutality. We find the “why”. We understand. It would be nice if this helped to prevent crimes in the future, but that probably isn’t the main reason we look for it: the main reason is that it lets us stop thinking about the terrible thing that has happened.

In the Letby case, the first question has always been whether something terrible did in fact happen. Were the deaths murders, or tragic misfortune? Murders, or collective incompetence? Without a narrative to anchor the idea of her guilt, there will always be just enough doubt to sustain these questions. Miscarriages of justice happen, more often than is pleasant to think. It is not ludicrous, given the nature of the evidence, to consider the possibility that this may be one of them.

Having absorbed as much of the evidence as I can bear to, I’m not one of them. The association between her presence and unexpected harm coming to babies seems too strong to be discounted, and alternative explanations such as ward-wide infections unproven. The picture of her guilt holds. But the bits that don’t fit still nag at my focus. Even after Letby’s diary was opened up to the court, she kept her secrets. As long as she remains impossible to understand, there will be an urge to change her story into something comprehensible.

A paediatrician, Roy Meadows, called as an expert witness for the prosecution, told the court that the probability of the two deaths happening by chance was one in 73 million: that is, 8,500 times 8,500.

As it happens, that’s not true. This calculation assumes that the deaths are entirely uncorrelated, but we know that SIDS can run in families and be affected by environmental conditions. If you have one case of SIDS in your household, while incredibly rare, you are more likely to have a second; the 73 million figure is orders of magnitude too high. But that wasn’t Meadows’s big mistake.

His big mistake was the following: he assumed that if the probability of the two deaths happening by chance was one in 73 million, then the probability that Sally Clark was innocent was one in 73 million as well.

But this is wrong. Crucially, catastrophically wrong. As wrong as assuming that because only one human in eight billion is the President of the United States, there’s only a one-in-eight-billion chance that the President of the United States is human.

Nonetheless, Meadows’s testimony helped convict Clark in 1999. She spent three years in jail before her conviction was overturned on appeal. Her life was, obviously, ruined. It will not surprise you to learn that she drank herself to death four years later, alone. It’s a haunting story.

The mistake made in Clark’s case is a subcategory of a far wider failure of reasoning, a failure I discuss in my new book, Everything is Predictable: How Bayes’ Remarkable Theorem Explains the World. But in legal circles it comes up again and again — often enough to have its own name: the “prosecutor’s fallacy”.

There was a recent article in the New Yorker about the nurse Lucy Letby, convicted of murdering seven babies in a neonatal ward. The online version is blocked in the UK, because of contempt-of-court laws, despite her appeal against the convictions being denied: she still faces a retrial on one count of attempted murder.

This piece is not about Letby. I do not know the facts of the Letby case and would not be allowed to write about them if I did; whether that is a strength or a weakness of British law, I leave to others to discuss. But I do know that courts, in the UK, US and elsewhere, have a bad history when it comes to the use of statistical evidence. To understand why, we need to go back to the work of an 18th-century nonconformist minister.

The Reverend Thomas Bayes’s eponymous theorem was published in a paper called “An Essay towards solving a Problem in the Doctrine of Chances” in 1763, five years after his death. Previous work in probability theory had answered the question: how likely am I to see some event, given a hypothesis? For instance, if we assume that my dice are fair, and I roll three of them, I can expect to see three sixes one time in 216. That’s called sampling probability.

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But most of the time, with statistics, we want to answer the opposite question: how likely is my hypothesis to be true, given some new event? That’s called inferential probability, and it’s a completely different thing.

Say I go to the doctor’s and I get a cancer test. It’s quite an accurate test: if I have cancer, it will correctly say so 99 times out 100; if I don’t have cancer, it will correctly say so 99 times out of 100. If I get a positive result, then, what’s the chance that I have cancer? Is it 99%?

No. The answer is you don’t know. At least, not with the information I’ve given you.

Imagine that this particular cancer is rare: only one person in 1,000 has it. You test 100,000 people at random. Of that 100,000, about 100 will have the cancer, and your test will pick up 99 of them. Of the remaining 99,900, your test will correctly say that 98,901 are cancer-free.

But that means that it will incorrectly say that 999 people do have cancer when they don’t. So of your 100,000 tests, 1,098 will come back positive, and only 99 of them are true positives. If you are one of them, then there is just a 9% chance you have cancer.

You can’t answer the question “How likely am I to have cancer, given this test?” without first answering the question “How likely did I think I was to have cancer in the first place?” That was Bayes’s big insight. You need what is called a prior probability. If the cancer was less rare, then your positive test would be more worrying: if one person in every 100 had it, then a positive result would mean about a 50% chance you have the disease.

It’s counterintuitive and weird. What do you mean, this 99% accurate test result is almost certainly wrong? But it is mathematically unavoidable.

You can probably see the bearing this has on statistical evidence used in court. Take DNA tests, for instance: you might do a DNA test on a sample from a crime scene. It matches a result on your database. There’s only a one in 3 million chance that someone’s DNA would match the sample by chance. Does that mean there’s only a one-in-3-million chance that your suspect is innocent? As you’ll realise by now, no it does not.

It depends on your prior probability. If your database is a random sample of the British population, then the prior probability that any given person is the culprit is one in 65 million. If you tested the whole population, you’d get about 20 matches just by chance.

But if you are a modern-day Hercule Poirot, and you’re only testing 10 people trapped by a snowstorm in a country mansion, then your prior probability is one in 10, and the chance it’s a false positive is about one in 300,000.

Real cases have turned on these details: a man called Andrew Deen was convicted of rape in 1990 on the basis of DNA evidence that an expert witness said there was only a one in 3 million probability of a chance match. His conviction was overturned — although he was found guilty again in a retrial — because a statistician pointed out that “How likely is it that a person’s DNA would match the sample, if they are innocent?” and “How likely is it that someone is innocent, given that their DNA matches the sample?” are very different questions.

In Sally Clark’s case, the problem was not testing, but clustering: two rare events happening simultaneously. But, again, the probability of seeing those events by chance is not the same as the probability that she was guilty.

Hers is far from the only case in which the use of statistics has raised suspicion. In 2022, the Royal Statistical Society wrote a report on statistical evidence in criminal trials, and noted that one of the most common reasons that medical professionals are accused of murders is because “an apparently unusual number of deaths occurs among their patients”.

But these cases are doubly difficult to evaluate, the report noted, because “they involve at least two levels of uncertainty”. As well as the normal uncertainty over whether an individual committed a murder, there is uncertainty over whether any murders occurred at all.

The Dutch paediatric nurse Lucia de Berk was convicted, in two trials in 2003 and 2004, of seven murders and three attempted murders of children under her care. A criminologist told her trial that “the probability of so many deaths occurring while de Berk was on duty was only one in 342 million”.

But even if that were the case — and again, it wasn’t; the RSS estimated that if you took into account all the relevant factors, the chance of seeing a cluster like that could be as high as one in 25 — that’s not the same as the chance that de Berk was innocent. In order to establish that, you would need to take into account the prior probability that someone would be a multiple murderer — a mercifully tiny chance. De Berk’s conviction was overturned in 2010, thanks in part to the work of Bayes-savvy statisticians.

Bayesian reasoning doesn’t only reveal the wrongly convicted — in some instances it could have led to the guilty being detected. During OJ Simpson’s trial for the murder of his wife and her friend, for instance, the prosecution showed that Simpson had been physically abusive. The defence, though, argued that “an infinitesimal percentage — certainly fewer than 1 in 2,500 — of men who slap or beat their wives go on to murder them” in a given year, so it wasn’t relevant to the case.

But this is simply the opposite mistake to the prosecutor’s fallacy. The probability that a man who beats his wife will go on to murder her in a given year might “only” be one in 2,500. But that’s not what we’re asking. What we want to know is, if a man beats his wife, and given that the wife is then murdered, what is the chance that he did it?

The scholar of risk Gerd Gigerenzer had a go at answering that. The base rate for murders among American women is about five in 100,000. Assuming the one-in-2,500 probability is correct, then of 100,000 women with abusive husbands, about 99,955 will not be murdered. But of the remaining 45, 40 will be murdered by their husbands. The correct probability that the husband did it should be nearly 90%. Bayesian thinking might have helped convict Simpson.

And although he does not use full Bayesian reasoning but a slimmed-down version, the insight that we should update our existing probability estimates with new information — cumulative monitoring, rather than one-off testing — could, argued the statistician Professor Sir David Spiegelhalter, have spotted both the catastrophe at the Bristol Royal Infirmary and the murders of Britain’s worst serial killer, Harold Shipman, earlier, and saved many lives.

The Lucy Letby case does not turn solely on statistical evidence, and I make no arguments about whether she is guilty or innocent here. But people, including juries, prosecutors and judges, have misunderstood probability in the past; Sally Clark and Lucia de Berk had their lives ruined by that misunderstanding. Thinking like a Bayesian might have helped prevent that.

When we encounter murderous healers, as we have in the cases of the British GP-turned-serial killer Harold Shipman, the homicidal American nurse Charles Cullen, and now Lucy Letby, the instinct is to cry “monster!” and make every effort to distance ourselves from such horrific behaviour. Surely these people are mentally ill? Psychopaths, deviants: people totally unrooted in basic human compassion.

In reality, this doesn’t stack up. Shipman and especially Letby seemed to be confusingly normal people living mundane lives. They were well thought of — if that is, they were thought of much at all. Shipman was a former rugby player who expressed remarkably compassionate and forward-thinking ideas, very much current today, about care for people with mental disorders. Meanwhile, Letby has been characterised as “beige” — not a word that springs to mind for many serial killers. She was a fan of salsa dancing, adopting cats, and taking holidays with her parents while in her mid-twenties — and was so “nice” that doctors initially discounted her as a possible cause of the deaths. The explanations of her motives: that she had a crush on one of the consultants; that she feared never having children of her own; that she suffered from Munchausen’s Syndrome by proxy or psychopathic tendencies, seem contradictory.

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In my many years of working with murderers, serial killers, and criminals with a range of complex mental troubles, I have learned that when things seem most confusing, the answer often lies in a person’s life experience, and especially in the “obstacles” they have tackled in their personal relationships. As psychoanalytic theory shows us, people are often poor narrators of their own reasons for doing things: instead, their actions tend to powerfully echo their life experiences. Harold Shipman, for example, at the age of 17 watched his mother die a slow and horrible death of lung cancer, eased only by the use of large amounts of morphine. This was exactly the method he later used to kill his own victims as they struggled (on the whole) with illnesses that they would have recovered from, something that as a GP he would have known.

When it comes to understanding this drive towards death, we must naturally start with Sigmund Freud. Freud believed that there were two “drives” in everyone’s unconscious: an Erotic drive towards pleasure, procreation and life; and a Thanatotic drive towards destruction, pain and death. Each is important because we all eventually have to face up to the temporality of sexual relationships and the inevitability of death. But unfortunately for those of us working in forensic psychiatry or psychotherapy, Freud wrote a great deal more about the Erotic than he did about death. This means that a lot of thinking about perversion — the turning away from life and towards death — has been filled in by other writers and psychotherapists since.

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Estela Welldon, a forensic psychotherapist, writes particularly powerfully about “dancing with death”; the idea that for some people a closeness to death provides a kind of life-affirming excitement, a reminder that they are still alive. It doesn’t matter whether this is their own death, as is the case for people who self-harm or consider suicide, or the death of others who choose to express this outwardly (and certainly not always fatally if we think of extreme S&M sex play). Very often, this takes the form of what Freud called a “repetition compulsion”: a compulsive desire to recreate important circumstances from one’s past in a way that induces the excitement of the “death dance”, but provides a sense of control over it. Shipman is an obvious example, but I have worked with many male offenders who cannot seem to help but repeat upon their loved ones the violence they saw inflicted on family members by fathers they swore they despised.

For Letby, her own birth was a “near-death” experience; she was told that her life had been “saved” by the nurses who helped her mother through a difficult childbirth. This has been cited as the main reason Letby went into nursing herself. But what was it that appealed to her so much? A sense of debt to the profession, possibly, although it must have seemed awesome to her that another individual had such power over life, and that she herself could take on this power.

As someone who trains aspiring psychologists and psychotherapists, I meet a lot of “wounded healers”: those who chose to follow such a profession because of their own life experiences. There is nothing wrong with this, in itself, and of course talking therapies equip people with considerably blunter tools for helping and harming than those wielded by doctors and nurses. Yet several ambitious people I have trained have subsequently left the caring professions because the realities of working every day with suffering people can often be more oppressive and harrowing than it can be life-affirming. Young women who seem hell-bent on deliberately harming themselves; middle-aged men with depression so crippling they can barely mumble a sentence in their clinic session; people from migrant backgrounds who can barely speak English, but their distress and trauma seem to seep from their very pores and cause your voice to choke in your throat. All of these cases, day after day, and some of it reminding you of — even triggering — your own past traumas and obstacles. We politely call this experience “compassion fatigue”.

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It shows great maturity to abandon your dream when you realise that your fatigue is so strong that you cannot continue. But I imagine that this must be even harder to do if you genuinely believe your life has been saved by the profession you have spent countless hours training in. In that case, where does this fatigue from a constant experience of death and near-death go? The natural, unspeakable response would be to try to exert some kind of control over it; to show that the life-affirming experience of her birth could be repeated through an equally potent act. But, lacking the skill to save truly sick babies, her only way to exert this power, to show that she still lived and her life was worth living, was to take the lives of others in her care. Quietly, and with minimal suffering, perhaps, but was this a choice of convenience rather than compassion?

As the respected forensic psychologist Naomi Murphy has said, to take life in this way is a callous, almost psychopathic response — but the uncomfortable truth is that it is also oddly human. Letby is not anything so straightforward as a monster or a psychopath. Confronted repeatedly with suffering and loss in a world she so fervently believed she belonged, her drive to sustain life became perverted into a drive to inflict death. Only in this way could she feel that her identity, her power — that instinct to care which, in others, kept her alive and then gave her a reason to exist — could be sustained.

It always astonishes me that while those of us working in mental healthcare are expected to receive regular, even daily, supervision and therapy to “detoxify” us from the psychic pain, those in physical healthcare — who, frankly, see far more death and trauma — receive no such consideration. In the face of suffering and death, which was already worryingly common at Countess of Chester Hospital long before Letby worked there, NHS workers are expected to simply get the job done. It feels unreasonable to suggest that, amid a staffing and funding crisis in the NHS, regular psychological support should be offered to those who regularly work with death. Yet at the same time, this would be one way to spot those for whom compassion fatigue had started to eat away at their empathy — and paralyse their drive to life.