Examining Locard's Exchange Principle


Be very, very quiet. Not because we’re in the middle of classes (well, maybe partly because we’re in the middle of classes) but because our Forensic Science students are on the prowl and discovering that Locard’s exchange principle is not a mystery but rather very, very real.

This principle maintains that the perpetrator of a crime will bring something into a crime scene and leave with something from it, and that both can be used as forensic evidence.  A pioneer in his field, Dr. Edmond Locard (1877-1966) formulated the basic principle that every person leaves a trace.  Trace evidence is any type of material left behind (or removed from) a crime scene as the result of contact between two surfaces (such as shoes and the floor, or fibers from a chair).

Our Saints traveled the hallways today in their stocking feet only to put Locard’s principle to the test. What remains to be seen is how much evidence they carried back to Ms. Bartlett’s classroom and what crime they may have committed….  Hmmm.  We’re leaving no stone unturned.



"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 fibres 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."