Getting to the truth in a statement has baffled criminal investigators for centuries. Regardless if it's an accident or a homicide, the burden of proof falls upon the State.
Many top law enforcers generally agree that eye-witness testimony is suspect. They also agree that most people want to tell the truth. but in many cases, what you hear may not be exactly what occurred:
• The individual may be telling you what they believe you want to hear
• The memory of the individual may be faulty
• The individual is hiding some fact, including their involvement in the incident.
Grilling a suspect was a normal practice performed by often well-meaning investigators. And in countless cases, a subject was coerced into giving a full confession simply to end the ordeal of the interrogation.
With its Miranda and subsequent landmark rulings, the U.S. Supreme Court forced a better degree of law and order on investigative bodies. "You have a right to remain silent... " should be the first words uttered from the mouth of an investigator, once an interview turns into an interrogation.
Among the many so-called truth-detecting methods employed during interrogations is "body movement." The interrogator makes visual note of how the suspect comports himself physically. Is he/she nervous, uncomfortable, irritable, twitching and a host of other body movements were, at one time, thought to indicate deception. But current training models now tell us that body movement of a subject may be entirely normal when placed under the stress of an official law enforcement-conducted conversation. Some research indicates that depending upon body movement as a measure of truthfulness is a "coin toss," 50% of the time you may be right!
Over a decade or more, the technique of Statement Analysis is used much more often. Statement analysis is best defined as: A "statement validity assessment", "content analysis", "investigative discourse analysis", and "scientific content analysis." It is a technique proponents claim can be used to detect concealed information, missing information, and whether the information that person has provided is true or false. (Wikipedia)
Statement Analysis involves the actual linguistic methods (specific words or phrases) an individual uses to describe an incident or to establish an alibi. The subject may leave obvious gaps in the narrative, gaps that actually included his direct involvement. And the subject may embellish the narrative by adding useless, inappropriate and meaningless chatter.
Statement analysis also includes a written account of the incident, but the investigator will still look for the same tip offs as to the veracity of what the subject writes.
The memory of the subject will also influence the completeness of the statement. The individual may be perfectly honest but is having a problem recounting the events as they actually happen. This is where the investigator employs the now well-accepted technique of cognitive interviewing. This technique was first introduced to law enforcement by an instructor for the National Transportation Safety Board (NTSB). This technique has been used countless times when victims and survivors of serious accidents involving mass transit conveyances (aircraft, trains, buses, etc.) are questioned about the accident.
Cognitive interviewing involves coaxing recall using techniques such as having the individual describe his activities from the time he awoke until the time seconds before the accident. It also uses a form of transference like, "Imagine yourself as a fly on the wall. What can you see? What is being said?"
Getting to the truth is no one-step procedure. Law enforcement heralded the invention of the Lie Detector or Polygraph by a medical student in 1921 at UC, Berkeley, but U.S. courts now prohibit the introduction of polygraph evidence as being unreliable.