Criminal Mischief: Episode #51: Forensic Science For Crime Writers: Evidence
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Locard’s Exchange Principle
Wherever he steps, whatever he touches, whatever he leaves, even unconsciously, will serve as a silent witness against him. Not only his fingerprints or his footprints, but his hair, the fibers from his clothes, the glass he breaks, the tool mark he leaves, the paint he scratches, the blood or semen he deposits or collects. All of these and more, bear mute witness against him. This is evidence that does not forget. It is not confused by the excitement of the moment. It is not absent because human witnesses are. It is factual evidence. Physical evidence cannot be wrong, it cannot perjure itself, it cannot be wholly absent. Only human failure to find it, study and understand it, can diminish its value.
Professor Edmond Locard (1877-1966)
Direct and Circumstantial
Eyewitness accounts and confessions are direct evidence, everything else (DNA, fingerprints, blood, hairs, fibers, bullets, etc.) is circumstantial
Physical and Biological
Physical evidence may take to form of fingerprints, shoe and tire impressions, tool marks, fibers, paint, glass, drugs, firearms, bullets and shell casings, documents, explosives, and petroleum byproduct fire accelerants. Biological evidence would be a corpse, blood, saliva, semen, hair, and botanical materials, such as wood, plants, and pollens.
Crime scene and other evidence serve many purposes in the arena of criminal investigation. These might be considered as:
Corpus Delicti---This is the “body” or the essential facts of the crime. Evidence will reveal exactly what type of crime was committed, such as robbery, murder, or a sexual assault.
Modus Operandi (MO)---This is the steps and methods the perpetrator employed to commit the crime. A criminal’s MO tends to be repetitive so that identification of his MO can help with uncovering, even trapping, the perpetrator.
Linkage—The association of linkage of a suspect to a victim, a place, or other pieces of evidence is critical to solving the crime.
Verification---Evidence can substantiate or refute suspect or witness statements and show who is lying and who is speaking the truth.
Suspect identification---Evidence can often identify the perpetrator. Fingerprints or DNA would be examples of such evidence.
Crime Scene Reconstruction---The evidence often allows investigators to reconstruct the sequence of events of the crime.
Investigative Leads---Evidence will frequently direct the lines of investigation followed by the police and the coroner and often lead them to the perpetrator.
Main Functions of Evidence
Identification and Comparison
The forensic analysis of evidence items is done for two main purposes: identification and comparison. Identification is done to determine what exactly a particular item or substance is. Is this white powder heroin or crystal methamphetamine or sugar? Who manufactured the shoe that left the print at the crime scene? Is this brown carpet stain dried blood or chocolate sauce? Are there petrochemical residues present in the debris of a suspicious fire?
Comparisons are done to see if a known and a suspect item or substance share a common origin. That is, did they come from the same person, place, or object? Did this fingerprint, hair, or blood come from the suspect? Does this paint smudge found on a hit and run victim’s clothing match that of the suspect’s car? Does the bullet removed from a murder victim match the one test fired from the suspect’s gun?
Class Versus Individual Characteristics
Some types of evidence carry more weight than do others. Hair and fibers can suggest, while DNA and fingerprints can absolutely make a connection. The difference is that some evidence shares class characteristics and others individual characteristics.
Class characteristics are those that are not unique to a particular object, but rather serve to place the particular bit of evidence into a specific class.
Individual characteristics are as close to absolute proof of the origin of the evidence item as is possible.
Reconstructive and Associative Evidence
Whether the evidence is class or individual in quality, it may be used to reconstruct the events of the crime or to associate a suspect with the crime scene.
Reconstructive evidence is any evidence that helps in reconstructing the crime scene.
Associative evidence is evidence that ties the suspect to the crime scene. In storytelling, evidence can be a Red Herring by appearing to be associative when it actually isn’t.
Primary and Secondary Crime Scenes— The primary scene is where the crime actually occurred, while any subsequent scenes are deemed secondary.
Staged Crime Scene— when the perpetrator alters the scene in an attempt to make the scene look like something it’s not.
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