Many law enforcement officers consider the majority of their cases as routine, even some death investigations. A seasoned detective may investigate a hundred or more self-inflicted shooting deaths. For investigators, crime scenes begin to take on a certain familiarity, and a quick assessment of any crime scenes with a firearm found next to the victim may result in the investigator assuming the death is self-inflicted. The observations and actions of observers are influenced by their expectations. Before the actual investigation is initiated, the conclusion can be inappropriately established. When this happens, there is often an unconscious tendency to seek evidence that supports that preexisting theory or opinion and disregarding evidence conflicting with the theory. Selectively focusing on evidence to support a conclusion, instead of integrating all evidence to form an impartial conclusion is called confirmation bias.
When confirmation bias is the basis for an investigation, officers may be tempted to cut corners in order to dedicate their efforts to cases they deem more important, more challenging, and even perhaps more worthy. Enemy of the Truth: Myths, Forensics and the Kennedy Assassination examines how officers may ignore proper procedure on a regular basis, thereby developing the mindset and behavioral habits of poorly structured investigative techniques. When an unusual or high-profile crime occurs, that same officer feels enormous pressure to complete an accurate and thorough investigation in a timely manner. That pressure creates stress that in turn results in working memory loss. With hampered cognitive abilities, the officer may revert to habitual behaviors and habitual cognitive choices.
Evidence suggests that the Dallas Police Department investigation of the Kennedy assassination was driven by confirmation bias stemming from the arrest of Lee Harvey Oswald. After all, the Dallas Police Department had their man; all they had to do was prove it.