“Every contact leaves a trace” is how the Locard Exchange Principle which is really a hypothesis is summarized.

It has been written as follows:

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.

The hypothesis goes that there are basically two types of physical evidence that come as a result of the Locard Exchange Principle:

  1. Gross Evidence
  2. Trace Evidence
forensic science trace evidence

Hairs are a type of trace evidence

forensic science locard exchange principle

When a hand touches wet cement is a good example of gross evidence

locard exchange principle in forensic science

By placing ones hands in wet cement, one leaves the imprint in the cement and exchanges for wet cement on one's hands

If one is willing to accept this hypothesis, then it becomes quite clear and evident that especially with trace evidence the possibility of contamination is high and great preventative measures must be taken by first responders and subsequent crime scene processors so as to not negatively impact the physical evidence at the scene.  This is why dedicated clean suits and the like are necessary when processing a scene.

dumb forensic science collection

Dumb forensic science collection

crime scene clean suit

Proper suiting up in a clean suit at a crime scene

The whole point is to try to have little to no impact on the scene or the collected evidence so as to not contaminate the scene or the evidence.


4 Responses to “The Locard Exchange Principle in Forensic Science: The real iTouch”

  1. Attorney Stewart Bersaw says:

    This site does a amazing job talking about this concept!

  2. Thank you. Consider joining in our facebook page and also consider trying to participate in our weekly forensic science geek of the week challenge.

  3. michelle says:

    teach HS Forensic Science
    you give me clear and useful information Thanks!

  4. Thank you Michelle! I do teach lawyers forensic science topics which the typical lawyer has about a HS education in science.

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