Tunnel vision occurs when investigators (or defence lawyers) pick a theory, or suspect, and build a case around that person (theory) while undervaluing evidence that contradicts that theory.
Correct investigation methods are based on following the evidence, regardless of where it leads.
Kohberger was arrested as a result of following the evidence. A specific white vehicle was seen circling the house on the night of the murders. CCTV footage was used to track that vehicle before and after the murders. Specific details about the vehicle (make, model, year, missing front licence plate) were used to narrow the suspect list. The vehicle was identified. The owner was investigated. DNA from the owner matched evidence (sheath) left at the murder scene. The item (related to murder weapon) containing DNA evidence was directly connected to the suspect.
Theories about alternate suspects are based on tunnel vision. That is, a suspect is chosen and a theory is concocted based on wrapping vague evidence around that suspect while ignoring evidence that does not support the theory.
"Tunnel vision in the criminal justice context can be described as a tendency of participants in the system, such as police or prosecutors, to focus on a particular theory of a case and to dismiss or undervalue evidence which contradicts that theory. This mental process leads to “…unconscious filtering in of evidence that will ‘build a case’ against a particular suspect, while ignoring or suppressing evidence respecting the same suspect that tends to point away from guilt.”
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Confirmation bias is a powerful psychological process that causes an individual to unconsciously prefer information that supports a conclusion that they have already settled on and to disregard or be overly sceptical about information that contradicts that conclusion."
Department of Justice Canada's Internet site
www.ppsc-sppc.gc.ca