Template:M intro crime tunnel vision

By tunnel vision, we mean that “compendium of common heuristics and logical fallacies,” to which we are all susceptible, that lead actors in the criminal justice system to “focus on a suspect, select and filter the evidence that will “build a case” for conviction, while ignoring or suppressing evidence that points away from guilt.” This process leads investigators, prosecutors, judges, and defence lawyers alike to focus on a particular conclusion and then filter all evidence in a case through the lens provided by that conclusion.

— The Multiple Dimensions of Tunnel Vision in Criminal Cases by Keith Findley and Michael Scott (2006)

To a man with a hammer, everything looks like a nail.

—Abraham Maslow

Prosecutor’s tunnel vision
/ˈprɒsɪkjuːtəz/ /ˈtʌnᵊl/ /ˈvɪʒᵊn/ (n.)
The collection of biases and cognitive gin-traps that can lead prosecutors — those who “prosecute” a particular theory of the world — to stick with it, however starkly it may vary from available evidence and common sense.

So named because it is often literal prosecutors, of crimes, who suffer from it. This kind of tunnel vision has led to notorious miscarriages of justice where innocent people come to be convicted notwithstanding clear and plausible alternative explanations for their ostensible “crimes”.

The same tunnel vision also motivates ideologies, conspiracies and management philosophy: 360-degree performance appraisals, outsourcing, the war on drugs; the worldwide AML military-industrial complex: are all cases where those “prosecuting” the theory stick with it even though the weight of evidence suggests it does not work and may even be counterproductive.

The “prosecutor’s tunnel” begins with clear but simplistic — misleading — models of a messy world. Humans have a weakness for these: we are pattern-matching, puzzle-solving animals. We are drawn to neatness. We resile from intractability as it indicates weakness: that our frail human intellect has been defeated by the ineffable natural order of things.

An elegant hypothesis

Sometimes the sheer elegance of a prosecutor’s case can crowd out common sense and the basic intuition that this cannot be right.

We have built our legal institutions to be vulnerable to this kind of crowding out. Criminal law proceeds upon data and the weight of evidence but disallows “intuition”. Hence, there is an asymmetry: evidence is better at saying what did happen than what did not. This is especially so where there is no direct evidence that the defendant actually did what she is accused of.

Circumstantial evidence does not directly implicate a defendant but is consistent with the prosecution theory. It accumulates: if there is enough of it, and none points away from the defendant, it can tell us something. But, correlation and causation: evidence that is “consistent with” a prosecution theory does not prove it: that JC owns a bicycle is consistent with his competing in the Tour de France; it does not make him any more likely to do it. Evidence can look more meaningful than it is. This is where intuition ought to be able to help us.

As it is, intuition’s role is relegated to underpinning the presumption of innocence. A prosecutor must prove guilt; the accused need not prove anything: she cannot be expected to explain what happened for the simple reason that and innocent person should have no better idea about it than anyone else. The jury, we hope, leans on its intuition when conjuring doubts.

Experience tells us otherwise. In what follows, JC takes three notorious cases from the antipodes to see what can happen when, with no direct evidence, those arguing the case become afflicted with tunnel vision, and intuition and common sense are relegated behind “data” and circumstantial evidence. Then we will look at what causes this condition.

A ring of dust around Ayers Rock

Lindy and Michael Chamberlain and their three children were camping at Ayers Rock in central Australia in August 1980. They were relaxing with other campers around a campfire when Lindy heard a disturbance near the tent where her infant daughter Azaria was sleeping. As she went to check on the baby she thought she saw a dingo running out of the tent. When she got to the tent, the child was gone.

Lindy raised the alarm at once. Azaria was never found.

Dingo attacks are rare, and the police believed Lindy was behaving strangely. Regarding the “dingo” explanation as absurd, quickly they concluded that Lindy had murdered and disposed of her baby. Their media management was such that before long, the public believed it, too.

The police built their case from the little positive evidence they had: Lindy’s absence from the campfire, her strange religious beliefs (the Chamberlains were Seventh-Day Adventists), her odd behaviour when interviewed and critically what appeared to be spattered infant blood in the footwell of the Chamberlain’s car.

But many aspects of the police case were highly implausible: Lindy was absent for no more than five or ten minutes, so logistically, it would have been hard to murder Azaria in the way proposed — with blunt scissors — dispose of the body and all evidence before returning to the campsite to raise the alarm. Never mind how unlikely it was for a mother — let alone a devout Christian mother — to murder her own infant in cold blood.

Nevertheless, in 1982, Lindy Chamberlain was convicted of Azaria’s murder. She spent three and a half years in prison before Azaria’s matinee jacket was found, four kilometres from the campsite, at the entrance to a dingo lair. Lindy was quickly released and pardoned, but her conviction was not finally quashed until 1992.

The “blood spatter” in the footwell of the Chamberlains’ Holden Torana turned out, much later, to be a standard sound-deadening compound applied during the car’s manufacture.

Satanic panic in the Garden City

In 1991, Peter Ellis, a childcare worker at a daycare centre in New Zealand was charged with horrific abuse of several preschool children in his care. Police alleged, on the children’s own evidence that, among other things, Ellis abducted children en masse during the day, took them across town to a flat where he subjected them to bizarre rituals and acts of unthinkable cruelty and violence before returning them to the creche unobserved before their parents were due to collect them at the end of the day.

In 1993 Ellis was convicted on 16 counts of child sex abuse against seven children.

But none of the allegations was true.

In total, police and social workers interviewed 118 children but presented evidence from just 20. They discarded evidence from children who did not report abuse and those whose claims were patently impossible (some claimed amputation of organs that they still possessed).

Therefore, the evidence put before the court — and fully disclosed to the defence — appeared more credible than it might have had it been viewed in the wider context. Police interview techniques may have encouraged the children to embellish their stories or make them up altogether.

Ellis, who died in 2019, was finally exonerated posthumously in 2022.

Murder in the family

At 6:45 am on the morning of June 20 1994, twenty-two-year-old David Bain returned from his Dunedin paper round to find his whole family had been shot dead. He did not discover this immediately: it was midwinter dark at that hour, in the deep south of New Zealand. Without switching on a light, David first went downstairs to put on a load of laundry. He later told police it was so dark he did not notice his father’s bloodstained clothes on the machine and inadvertently washed them with his own, obliterating key evidence that might have exonerated him.

Returning upstairs, David discovered his father Robin lying in the living room beside a .22 rifle with a bullet in his head. He then found his mother, two sisters and younger brother Stephen all dead in their bedrooms. Stephen appeared to have put up a fight.

There was a note typed on the family computer. It said, “You were the only one who deserved to live.”

David placed an agitated call to emergency services. It was recorded and remains a part of the public record.

David told police his father, motivated by a troubled relationship with the family, must have murdered the family before typing the note on the computer to the absent David and turning the gun on himself.

Notwithstanding David’s account, based on circumstantial evidence including bloodstains on his clothing and spectacles, his fingerprints on the rifle, abrasions on his body consistent with a struggle with Stephen, and the dearth of physical evidence pointing to his father, David was convicted of all five murders.

David maintained his innocence. Joe Karam, a former New Zealand rugby international, led a campaign to challenge David’s conviction. It uncovered procedural shortcomings, inconsistencies and oversights in the police investigation and handling of evidence.

Twenty years after the original trial, the Privy Council quashed Bain’s convictions and ordered a retrial. After the second trial, David Bain was acquitted of all charges. He remains a free man.

Narrative biases

These cases illustrate the problem of relying on circumstantial evidence: with no independent direct evidence, one tends to start with a hypothesis and fit whatever secondary and forensic evidence you have into it, discarding whatever does not fit. This is the classic tunnel vision scenario. It can afflict those who would defend suspects just as firmly as those who prosecute them.

All kinds of theories circulated owing to the Chamberlains’ unusual religious beliefs and “odd behaviour” in the aftermath of Azaria’s disappearance. But devout Christianity is hardly a solid prior indicating a tendency to murder. Nor is “odd behaviour” in the aftermath of a mother’s most extreme psychological trauma. Who would not behave oddly in those circumstances?

That anyone could bring themselves to cold-bloodedly murder a nine-week-old baby is hard to imagine. Statistically, it is highly improbable. That the child’s own mother would is, in the absence of compelling evidence, preposterous. To even start with this theory you must surely have compelling grounds to believe it over all other possibilities — if not credible eye-witness evidence, then a documented history of violence, behavioural volatility or psychiatric illness grave enough to overthrow the strong human instinct to protect vulnerable infants. Lindy Chamberlain had no such history.

If there is any plausible alternative explanation for the baby’s disappearance, there must have been a reasonable doubt. It need not be more probable than the prosecution case: just not out of the question. Lindy Chamberlain provided one: a dingo snatching the child might have been unprecedented, but it was possible. There were dingoes in the area. They are predators. They are strong enough to carry away a human infant. A dingo was no less likely than a new mother noiselessly murdering her own infant just yards from a group of independent witnesses. That ought to have been the end of it.

Likewise, what Peter Ellis was alleged to have done is extraordinarily improbable. There are few documented cases of ritualistic abuse on that scale anywhere in the world. There are none in New Zealand. For such a thing to have happened without any prior evidence of such behaviour, with no adult witnesses, no one noticing the absent children and for none of the children to bear any trace of their supposed injuries makes it even less likely.

And there was a plausible alternative: nothing happened at all. All that was required for that to be true was for preschool children, perhaps at the prompt of interviewers already in the grip of prosecutor’s tunnel vision, to make things up. By comparison with “untraceable, unwitnessed, wide-scale ritual satanic abuse”, “children exercising their imaginations to please adults” is not improbable.

It is different for David Bain. While it is true that familicide is extremely rare and, therefore, absent prior evidence, highly improbable, there is no question that the Bain family were murdered. The only question was by whom.

On David’s own theory, only two people could have done it: his father and himself. It was, therefore, definitely familicide: the abstract improbability of that explanation is therefore beside the point. The probability that David was responsible is therefore greatly higher: before considering any further evidence there is a 50% chance he was responsible.

And a lot of the further evidence pointed in his direction. To not be the murderer, on his own evidence, David would have been extremely unlucky — forgetting to turn on the light, inadvertently disposing of exculpatory evidence, having incriminating injuries he could not explain — while no such evidence pointed to Robin. David’s defenders had their own tunnel vision, focusing narrowly on the provenance of each piece of incriminating evidence, identifying formal shortcomings in its value as evidence: questioning the manner of its collection, the chain of custody, raising possibilities of innocent explanations without evidence to support that alternative, and disregarding the wider context of the whole case.

Now, David Bain was acquitted of all charges. On the evidence, the jury could not rule out the possibility that Robin Bain was responsible. Not being satisfied beyond reasonable doubt that David was the perpetrator, he was correctly acquitted at law. But it remains likely that David was the perpetrator. As a piece of judicial procedure, the comparison between Bain’s case and those of Ellis and Chamberlain is stark.

Upstanding citizen turns master criminal. 1Does well.

Prosecutor’s tunnel vision cases often involve hitherto law-abiding citizens suddenly committing fiendish crimes without warning, explanation, motive, and without leaving direct evidence behind to implicate themselves (beyond an absent alibi).

Now JC is, ahem, told that committing violent crime without leaving any incriminating evidence is extremely hard. Especially in a controlled environment like an infants’ daycare centre or a hospital.

To be sure, serial criminals can operate in these environments — Harold Shipman is one — but they need to be meticulous in their preparation and method: over time they will hone their techniques and perfect a modus operandi, acquiring a ghoulish sort of metis. It is not the sort of thing that lends itself to opportunistic, freestyle offending.

Especially not from “beginners”. Nor should we expect flawless execution from first-timers. As with any complicated discipline, one learns as one goes. We should expect those who are new at serial murder to slip up, leave giveaways and clumsily trigger red flags. From new criminals we should expect “smoking guns”.

So their absence should tell us something. Especially where there is a contiguous absence of historical offending, a propensity for violence or any motive. All of these absences reduce the “prior probability” of foul play on the part of this suspect. Where a crime has certainly been committed, this goes only to who the perpetrator is. It may (as in David Bain’s case) be a small universe of credible offenders, all of whom have comparable prior histories, in which case that prior history might count for little. But if t he e universe of potential offenders is large, or if it is plausible that no crime was committed at all, an absence of incriminating evidence together with a lack of motive, history and propensity — in the absence of directly incriminating evidence, remember — tells us something “circumstantial”.

For Lindy Chamberlain there was no such incriminating evidence and a plausible alternative explanation. So too for Peter Ellis. But for David Bain it was different.

Standards of proof

The prosecution’s standard of proof is, in theory, high. Courts have discarded the time-honoured phrase beyond reasonable doubt and moved to dumb it down: these days, juries are directed to convict only if they are “sure”. This is meant to mean the same thing, but not all are persuaded that is how juries understand it.

There is some reason to think that juries start with an ad hoc presumption that any defendant put before them is somewhat likely to be guilty — perhaps to the “balance of probabilities”: if the police were competent and acted in good faith, why else would the defendant be in the dock?

Where there is only tendentious data supporting guilt and a total lack of “data” supporting innocence — what evidence could there be that you did not do something that did not happen? — there are grounds for confusion here.

Lindy Chamberlain was convicted of her own daughter’s murder, with a pair of blunt scissors, on the (tendentious) evidence of blood in the footwell of the family car.

The intuition that “a sane mother is most unlikely to brutally murder her own nine-week-old child at all, let alone with an improvised weapon and without warning or provocation” was not before the court. The jury did not hear this. Somehow the jury was persuaded not just that she did murder her child, but that there was no plausible alternative explanation for the child’s disappearance.

The three phases of prosecutor’s tunnel vision

So what is “prosecutor’s tunnel vision”, then and how does it come about? JC draws upon The Multiple Dimensions of Tunnel Vision in Criminal Cases by Keith Findley and Michael Scott in the Wisconsin Law Review (2006) and to a smaller extent Robert Cialdini’s Persuasion. The lessons are two-fold:

We are not as rational as we like to think and
Data is never the whole story.

Tunnel vision may afflict those with any kind of a “conviction” opinion: the behaviours strongly resemble those of people holding passionate religious and political persuasions. Once they take root, they are not easily displaced. If we treat prosecutor’s tunnel vision as a type of “emotional conviction”, we can see better how “prosecutors” can be so energetic in defending a bad mental model.

Tunnel vision has three phases: first, the enabling background conditions that make us vulnerable to tunnel vision; second, the pathways into a given tunnel; third, the cognitive biases that keep us there.

Background conditions

Certain dispositions, biases and miscellaneous psychological tics come together to create the conditions for tunnel vision to swamp an earnestly-held narrative:

The “anchoring” effect

When making decisions we “anchor” our expectations on the first information we get. We then recalibrate not against an abstract measure of value, but by reference to our original anchor. Our anchor disproportionately influences our assessment of subsequent facts.

Overconfidence about adjacent fields

Subject matter experts tend to overestimate their judgment of subtle problems, especially those adjacent to their expertise and the overall significance of matters within their expertise. Overconfident subject matter experts are less likely to consider alternative models, explanations, theories or evidence, let alone contradictory ones.

“To a man with a hammer...”

You don’t enlist in the army hoping never to shoot a gun. The police show up for work to detect crime, and prosecutors to prosecute it. They are primed this way, as are we all: to be useful; for their roles to be important and to make a difference. Our aspiration is to find a crime to prosecute. Those who would deter us from that stance may seem like part of the problem.

We tell ourselves archetypal stories: the dogged cop who sticks at her hunch despite Sarge’s threat to see out her career issuing parking tickets and overcomes the shadowy malign forces to save the day for righteousness and the American way. This is how the lead prosecutor of the Central Park Five saw herself.

We do not tell ourselves stories where we are conspiracy-obsessed geeks who are hounding innocent citizens to the electric chair.

Role pressure

Law enforcement agencies will be under pressure to get results. This may lead to missed opportunities: notoriously, despite repeatedly interviewing him, police overlooked Yorkshire Ripper Peter Sutcliffe, a Yorkshireman, because his accent didn’t match their profile of a man with a Wearside accent, based on a voice recording which turned out to be a hoax.

Getting there

Once you are anchored, equipped with a hammer and have started wandering around the house looking for nails, there should still be scope to falsify your operating theory. But again, psychological biases can override the dispassionate application of cool logic.

Extrapolation

Subject matter experts are prone to extrapolate. We are natural problem-solvers and model builders. We slip easily beyond our brief and intrude into matters where we have little experience. A statistician can give us a compelling account of the Bayesian probabilities, the forensic question of who or what caused the excess toxins is a question of forensics and not something a statistician has a better idea about than anyone else.

Precedent: bad heuristics

The greater the expertise, the more grooved the expectations, the stronger the heuristic, the greater our temptation to take that shortcut and presume that this is “one of those” cases. Where it is, the heuristic is a handy shortcut to the right answer. In the rare case where it is not, it can be dangerous. Heuristics are excellent, time-saving devices but when the model is wrong they can lead to trouble.

Base rate neglect

Lawyers are not natural statisticians. “Base rate neglect” — the “prosecutor’s fallacy” — is the natural tendency to ignore the “base rate” of a phenomenon in the population and instead focus on “individuating” information that pertains to the specific case in front of us. Here Lindy Chamberlain and Peter Ellis can be contrasted with David Bain.

Be careful with statistics about extremely rare events.

Say a certain test gives a correct result 999 times out of 1,000. That is, it only yields one “false positive” result in a thousand tests. It is tempting to assume that a positive result is conclusive. But if the general rate in the population of the condition it is testing for — the “base rate” — is only one case in 100,000 then for every single true positive result, you should still expect 100 false positives.

The same principle holds for criminal offending: if there is, say, a 1/342,000,000 chance that “so many suspicious deaths could have occurred with the same nurse on duty by sheer chance” and one nurse was in fact on duty for all those suspicious deaths, it may seem utterly damning. But this is to ignore the base rate of how many people are there in the world who murder multiple patients?

On a planet with eight billion people, for the odds of our nurse being a serial killer to as high 1/342,000,000 there would need to be twenty-three. There do not appear to be that many hospital serial killers in the last generation.

Yet “one in three-hundred and forty-two million” was the figure that convicted Dutch nurse Lucia de Berk of serial murder. Had this estimate been correct (it turned out to be a wild underestimate) sheer chance was still a likelier explanation.

Confirmation bias

All observation is theory-dependent: scientists must first have a theory before they can gather evidence to test it: otherwise, how do they know what evidence to look for?

Having picked a bad heuristic, we tend to seek out, interpret, and best remember information that confirms it. We may overweight evidence that supports our theory and disregard or minimise anything that contradicts it.

Convinced that Lindy Chamberlain had murdered her infant daughter Azaria, Police searched high and low for blood, eventually finding something resembling infant blood splattered over the footwell of her husband’s car. It turned out to be sound-deadening material containing iron oxide.

Evidence when taken in the abstract, would tend to “be consistent with” innocence can, with confirmation bias, be presented as damning. The innocent defendant who maintains innocence notwithstanding his wrongful conviction is presented as callous, lacking remorse and may expect a harsher sentence and less chance of parole. What is a wrongful convict meant to do?

An innocent defendant who, during a period of misfortune for which she was not responsible, went out with her friends, posting pictures of herself as a vivacious young woman who enjoyed socialising — is portrayed as thereby a callous, calculating demon. But this are exactly the behaviours would be what is you would expect from an innocent person. This ought to point away from the defendant, though with confirmation bias it points towards her.

Selective information processing

Focusing on certain pieces of evidence while ignoring others. Prosecutors might only present evidence that strengthens their case and neglect exculpatory evidence that could help the defence. Peter Ellis’ prosecutors interviewed one hundred and eighteen children. Many denied all suggestion of abuse. Some gave plainly preposterous accounts of what went on. They were not called to give evidence. Their statements do not appear to have been fully disclosed to the defence.

Groupthink

Thinking or making decisions as a group in a way that discourages creativity or individual responsibility: Prosecutors might conform to the prevailing opinion within their office, stifling dissenting views and critical analysis of the case. See also Dan Davies’ idea of the “accountability sinks” within an organisation. The Post Office Horizon IT scandal is perhaps the archetypal example of groupthink amongst a group of prosecuting lawyers.

Reductionism

Immersing into technical details that, by themselves and shorn of all context, seem to lead to one conclusion — especially one to which you are already anchored — notwithstanding the wider picture making the hypothesis unlikely. Especially in cases with no direct evidence, there is a great risk of this.

Staying there

Once we are in a tunnel, there are cognitive biases that prevent us from finding our way out.

Hindsight bias and the reiteration effect

In hindsight, people tend to think an eventual outcome was inevitable or more likely than they might have before it happened. “What is the chance that that nice woman you met at the campsite just now will, in three hours, brutally murder her own nine-week-old infant?” versus “Given that this woman’s nine-week-old infant has disappeared from the campsite, and the police suspect her of foul play, what is the change that she brutally murdered her own child?”

Through “hindsight bias” we project knowledge of actual outcomes onto our knowledge of the observed behaviour in the past, without realising that the perception of the past has been tainted by the subsequent information.

Once a person becomes a prime suspect hindsight bias suggests that, upon reflection, the suspect was the inevitable and likely suspect from the beginning. Evidence is malleable in light of this “realisation”.

There is also a “reiteration” effect. Our confidence in a theory increases the more we hear it, independent of its validity. The longer that police, prosecutors and witnesses live with the conclusion of guilt, the more invested in it they are, the more entrenched their conclusion becomes, and the more obvious it appears that all evidence pointed that way from the very beginning. It is increasingly difficult for police and prosecutors to consider alternative perpetrators or theories of a crime.

Outcome bias

Like hindsight bias, “outcome bias” involves projecting subsequent “outcomes” onto observed behaviour, only about the quality of a suspect’s decision. Subjects are more likely to judge as bad a suspect’s decision to operate when they are told the patient died during surgery than when told the patient survived.

This is the operator error presumption from Sidney Dekker’s The Field Guide to Human Error Investigations

Sunk cost fallacy

The inclination to continue an endeavour once money, effort, time or credibility has been invested, even when new evidence suggests the defendant might be innocent. (see also commitment when talking about persuasion)

Antidotes

Q: How many psychiatrists does it take to change a light bulb?
A: Just one; but the light bulb really has to want to change.

Some strategies to counteract the effect, but the predominant one is to want to keep an open mind.

Hanlon’s, and Otto’s razor

“Do not attribute to malice things that can just as well be explained by stupidity.”

—Hanlon’s razor

Don’t assume malice where stupidity will do; likewise, per Otto’s razor, don’t attribute to virtue something that could equally be attributed to self-interest; or to skill something that could equally be attributed to dumb luck.