Jarryd Bartle

‘Tunnel vision’ describes the process by which investigators, prosecutors and forensic experts focus on a suspect too early in the process, filtering evidence in ways that build a case for conviction while disregarding or minimising information that points towards innocence.

This phenomenon not only undermines the integrity of individual cases but also represents a systemic threat to the accuracy of the criminal justice process. The following outlines the core features of tunnel vision, known causes and possible strategies to mitigate falling into this trap during a criminal investigation.

Defining Tunnel Vision

Findley and Scott (2006) define tunnel vision as the tendency of investigators to focus on one suspect, choosing to select and interpret evidence in ways that confirm guilt, while suppressing or ignoring contradictory evidence. This ‘tunnelling’ results in a process where initial errors are left uncorrected, producing further investigative mistakes and systemic failure.

Tunnel vision is therefore not simply a matter of deliberate misconduct but is often the product of heuristics and cognitive shortcuts. By narrowing the scope of inquiry, it dismantles key safeguards intended to prevent wrongful convictions following flawed investigations.

Causes of Tunnel Vision

Several factors contribute to the emergence of tunnel vision. Firstly, a personal history with suspects or stereotyped perceptions by investigators can increase the odds that a suspect is narrowed in on early. Suspects who are seen as deviant, difficult or “odd” often attract disproportionate suspicion.

Further, high-profile cases generate community pressure that may lead to a premature focus on a single narrative. This is particularly the case when police, politicians or the media feel compelled to demonstrate rapid progress.

Limited investigative resources may also encourage the prioritising evidence that appears consistent with the preferred theory, leaving alternative leads unexplored. For example, because investigators often face strict time pressures, budget constraints, and competing case demands, they may allocate resources to building a case against the main suspect rather than expending scarce time and funds pursuing less certain or more complex lines of inquiry.

Another contributing factor is the potential for noble cause corruption, where investigators may justify overlooking exculpatory evidence on the grounds that pursuing the suspect is in the broader interest of community safety. While such reasoning may appear defensible in the short term, it corrodes the reliability of the system by subordinating procedural fairness to outcomes.

Prosecutors, too, are vulnerable to tunnel vision. In the United States, Schoenfeld (2000) have described prosecutors as the most powerful actors in the criminal justice system. In Australia, their power is more diffuse, but they nevertheless play a critical role in determining which cases proceed to trial.

Problems arise when prosecutors see themselves as hired guns, tasked with winning cases, rather than as ministers of justice with a duty to ensure fairness. Accepting the police case at face value, rather than subjecting it to independent scrutiny and allowing investigative errors to become entrenched.

The Role of Cognitive Biases

Tunnel vision does not occur in isolation but interacts with broader cognitive biases (systematic errors in thinking that occur when individuals process and interpret information through mental shortcuts).

Confirmation bias, the tendency to interpret new evidence in ways that reinforce existing beliefs, is particularly significant in influencing the trajectory of investigations. For example, the discovery of a suspect’s DNA at a crime scene may be viewed only as consistent with guilt, rather than also being consistent with narratives consistent with innocence.

Hindsight bias compounds the problem. Once an outcome is known, events leading to it are often regarded as inevitable, creating false certainty about investigative choices. Similarly, outcome bias leads investigators to rationalise decisions retrospectively. An admission of guilty, for instance, may be taken as proof that it was correct to ignore alternative suspects, even if the admission itself is questionable.

Together, these biases entrench the investigative narrative, making it difficult for alternative explanations to be considered, even when contradictions emerge.

Strategies to Reduce Tunnel Vision

Several strategies have been proposed to mitigate tunnel vision, these include:

• Blind witness interviews, where the interviewer has no knowledge of the suspect, reducing the risk of leading questions or suggestive procedures (Rivard, 2014).
  
• Designated “devil’s advocates” within an investigative team to deliberately challenge investigative assumptions (MacFarlane, 2008; Salet and Terpstra, 2014).
  
• Implementing processes where investigators actively consider counter-arguments nd alternative explanations as part of the investigative process (O’Brien, 2009)
  
• In forensic science, the technique of linear sequential unmasking has been developed to ensure that examiners are exposed to potentially biasing information only after forming their initial judgments (Dror et al., 2015; Krane et al., 2008).
  
• Finally, critical reviews of completed or unsolved cases can reveal patterns of error that would otherwise remain hidden (Rossmo, 2016).

Ultimately these strategies share a common aim: to consistently test the strength of the evidence gathered as part of an investigation, and to ensure all leads are considered before closing around a single hypothesis.