Locard's Exchange Principle


Whenever two objects come in contact with each other,

 there is a two directional transfer of material that takes place between them.

Edmond Locard (1877-1966) found that the contact between two objects results in the transfer of materials from one surface to another. The material that was picked up continues to be deposited on each additional surface contacted until there remains no more of that material to be deposited. When a person steps in mud, the mud, in decreasing amounts, continues to be deposited with each step until the shoe no longer has any mud on it. The decreasing amount of deposit indicates the direction of travel.

The transfer of materials is the basis for many of the forensic sciences. The examination of traces of deposits helps investigators to locate the source from which they came. They form connections and relationships which must be identified and interpreted.

Students will experience and observe the process by which trace materials move from place to place first hand. Once they have samples to examine, they will learn to describe their finds and try to determine what events occurred to produce the evidence they recovered.

      Vocabulary Terms

Circumstantial Evidence  materials or testimony which supports a case, but does not of itself provide proof

Deposit Location  the place where a material is transferred to by contact or other means of distribution

Exhibit  an item of physical evidence presented for consideration in court

Expert Opinion  a conclusion or interpretation reached by a highly trained or skilled person (an expert witness) based on the examination of evidence

Forensic Science  a scientific study suitable for use in the courts or in debate as it relates to the analysis of physical evidence

Material  any substance present in an environment which is identified by its physical properties

Physical Evidence  an actual material object that has some relevance to the case by providing a link or establishing a fact or proof

Source   the location or original object from which a material in whole or in part comes from

Testimony  the statements of a witness provided under oath used to determine the truth in a legal matter

Value  the actual worth of either an object as physical evidence or testimony as it relates to the case

Voir Dire  an inquiry by an attorney or the Judge to determine the qualifications of an expert witness

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