LOCARD'S THEORY
or
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
It is interesting when you think about it, isn't it? To the Criminal Investigator and Criminal Defense Investigator it is more than interesting, it is HIGH VALUE! And it works well on both sides. Locard's Theory is being proven to be correct more and more as technology advances. And you are about to find out how this can help you, if you are a Criminal Defense Investigator.
In all criminal cases, where the perpetrator of a crime has entered the scene of the crime when committing the crime, there is always an exchange between the perpetrator and the crime scene and/or the victim, if there is a victim. Crime Scene Investigators work from this principle in every case. Crime Scene Investigators look for trace evidence left behind by the perpetrator, and once they have a suspect they look for trace evidence that was picked up at the scene of the crime by the suspect. If Crime Scene Investigators can find physical evidence in either direction, they can connect the suspect to the scene of the crime and/or the victim of the crime.
In the circumstance that a Private Investigator is hired by the defense, it is a little harder to gather evidence, as he or she is usually coming in after the fact. However, it never hurts to do a crime scene walk through and be observant for evidence that could have been missed. There are many cases in which Crime Scene Investigators have missed evidence that was later found to exonerate a defendant.
Another use for Locard's Theory is in interviews with suspects that were developed by the defense. If you can accurately articulate Locard's Theory to a suspect, you can really get him or her at a very clear and distinct psychological disadvantage. This is perhaps one reason you should study and memorize the quote from Professor Edmond Locard above.
There is another value to Locard's Theory that may be helpful to the Criminal Defense Investigator in assisting the attorney they are working for. I would recommend that the Private Investigator that is getting good "court room time" (as all REAL Criminal Defense Investigator's should be), listen to the Prosecutor in their cases and note which cases the Prosecutor argues Locard's Theory to a Jury. This way they may be able to give the defense attorney a better argument against the Prosecutor in cases that the Prosecutor tries to convince a Jury that the lack of physical evidence should not be considered due to the fact that the Prosecution's Investigators were unable to gather any physical evidence. The Criminal Defense Investigator should note the case, case number, and as accurate of a quote as to how the Prosecutor phrased this argument as possible.
Locard's theory can work just as well for the defense as it can for the prosecution.......
Ricky B. Gurley