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{{d|Prosecutor’s tunnel vision|/ˈprɒsɪkjuːtəz/ /ˈtʌnᵊl/ /ˈvɪʒᵊn/|n}}
{{drop|T|he 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”. 

{{quote|
By tunnel vision, we mean that “compendium of common [[Heuristic|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)}}

{{drop|T|he same tunnel}} vision also motivates ideologies, conspiracies and management philosophy: 360-degree [[performance appraisal]]s, [[outsourcing]], the war on drugs; the worldwide [[Anti-money laundering|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===
{{drop|S|ometimes 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|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.

{{gbullet|Case study: [[Lindy Chamberlain]]<li>Case study: [[Peter Ellis]]<li>Case study: [[David Bain]]}}
====Narrative biases====
{{drop|T|hese 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 [[Prosecutor’s tunnel vision|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 [[Bayesian prior|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 [[Bayesian prior|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.<ref>Christchurch Journalist Martin Van Beynen’s fantastic podcast ''{{plainlink|https://interactives.stuff.co.nz/blackhands/not-guilty/|Black Hands}}'' compellingly makes this case.</ref> As a piece of judicial procedure, the comparison between Bain’s case and those of Ellis and Chamberlain is stark. 

====Tunnel vision and circumstantial evidence====
{{drop|W|here there is}} reliable [[direct evidence]] — eyewitnesses, recordings, and causative links between a suspect and the allegation — there is little need for inference; the evidence speaks for itself. But cases comprised predominantly of [[circumstantial evidence]] — that therefore depend on inferential reasoning — are vulnerable to tunnel vision because the complex of cognitive biases that make up [[prosecutor’s tunnel vision]] affect the process of inference. 

====Upstanding citizen turns master criminal. Does well.====
{{Drop|P|rosecutor’s tunnel vision}} cases often involve hitherto law-abiding citizens suddenly committing fiendish crimes without warning, explanation or motive. 

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 but they will need to be ''good'': meticulous in their preparation and method. Over time, they will hone their techniques and perfect a ''modus operandi'', acquiring a ghoulish sort of ''[[expertise]]'' in murder: killing patients in a closely monitored, controlled environment populated by trained experts hardly lends itself to opportunistic, freestyle offending. Hospitals, in particular, overflow with specialists who can detect subtle clues that ordinary laypeople — and burgeoning criminals learning their craft — have no idea about.

As with any complicated discipline, one learns as one goes. We should not, therefore, expect “beginners” to perform like master jewel thieves, slipping in and out, striking in the dark and leaving no trace. They will blunder. They will be careless. ''They will leave evidence''. They will slip up, leave giveaways and clumsily trigger red flags. From new criminals, we should expect “smoking guns”. 

So if a strange confluence of events is accompanied by ''no'' smoking pistol, this too has some prior probability value.  It does not ''exclude'' the possibility of foul play, but it does make it ''less likely''. 

People do not often flip, overnight and without warning, from conscientious citizens to compulsive criminals. If they did, we would ''notice'' it.<ref>They might snap into a sudden orgy extreme violence — but this plays out as desperate, meltdown ''mass'' murder, not calculated ongoing ''serial'' murder, and there is generally no doubt that it is murder and no shortage of [[direct evidence]] implicating the accused.</ref> When hitherto law-abiding people do slide into criminality, there is generally motivation, a history of antisocial behaviour, identifiable psychological trauma, drug dependency, observed personality change over time or diagnosed mental illness.<ref>Mental illnesses having a clear medical pathology, not suspiciously made-up ones out of ex- post facto symptoms like “Munchausen by proxy”. See the “circular correspondence bias” discussion below.</ref> Often ''all'' of these things. (Let us call them “[[Criminal propensity|criminal propensities]]”.)

The absence of ''any'' of criminal propensities in a suspect’s makeup should ''reduce'' the “prior probability” of foul play by that suspect. As we will see, “circular correspondence bias” can take such a ''lack'' of criminal propensity and somehow invert it into confirmation.

Where a crime has certainly been committed, this goes only to ''who'' the perpetrator is. There may (as in David Bain’s case) be only a small universe of credible suspects. If ''all'' “possible suspects” have the same lack of criminal propensity, it will count for little. But if the universe of “potential suspects” is large — or if it is plausible that ''no crime was committed at all'' — an individual’s lack of any criminal propensity should tell us something “circumstantial”.

Neither Lindy Chamberlain nor Peter Ellis had any criminal propensity and both cases there was a plausible alternative explanation. For David Bain it was different.

====Burden and standard of proof====
{{drop|T|he [[burden and standard of proof|''burden'' of]]}} proof is a different thing to the ''standard'' of proof. The burden is who has to prove their case: this falls squarely on the prosecution. The defence is not required to prove anything, least of all the accused’s innocence. 

But there is tension between that crystalline legal theory and the practical reality: it is in the defendant’s interest that ''someone'' casts doubt into jurors’ minds. Since the Crown plainly won’t be doing that, the defence must either rely on jurors to confect plausible doubts by themselves, or it must plant some doubts there. It is a brave defence counsel indeed who puts her client’s future in the hands of a jury’s imagination and capacity for creative thought.

All the same, the prosecution’s ''standard'' of proof — what it must do to discharge its burden of proof — is, in theory, ''extremely'' high. Courts have dumbed down the time-honoured phrase ''[[beyond reasonable doubt]]'': these days, juries are directed to convict only if they are “''sure''”. This is meant to mean the same thing, but not everyone is persuaded that is how juries understand it.<ref>{{Plainlink|https://www.newlawjournal.co.uk/content/dreaded-questions-doubtful-answers-the-trouble-with-sure-|New Law Journal: The Trouble With “Sure”}}</ref> 

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: if the police were competent and acted in good faith, why else would the defendant be in the dock? 

So where there is only ''tendentious'' data supporting a defendant’s guilt but a total ''lack'' of “data” supporting her innocence —  what evidence could there be that you did not do something that did not happen? — there are grounds for confusion here, and there is good evidence that juries do indeed get confused. 

Lindy Chamberlain was convicted of her own daughter’s murder, with a pair of blunt scissors, on the circumstantial evidence of what looked like blood sprays in the footwell of the family car.<ref>In fairness the crown submitted expert forensic analysis entered that it was specifically infant blood, so you can hardly fault the jury here. You can fault the crown forensics team though: it turned out to be acoustic deadening spray and not blood of any kind!</ref> 

Evidence supporting 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. What evidence could there be of that? 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. This was largely thanks to the strange collection of cognitive biases to which the prosecution had succumbed.