LOCARD’S EXCHANGE PRINCIPLE by Michael Rigg

Edmond Locard (1877-1966)

No doubt, you’ve seen or heard the saying, “Take only memories, leave only footprints.” It’s a shorthand way of telling humans to minimize their impact on nature. If you see something, leave it for someone else to see and enjoy. If you eat a Milky Way, don’t throw away the wrapper. Take it with you.

What a wonderful sentiment. What an exceptional goal. But to those of us who write mysteries, especially those involving crimes, it’s sheer, absolute, and utter nonsense. Pure baloney. Leave only footprints? Give me a break.

Those of us “in the know,” accept the “Gospel According to Dr. Locard.” Developed by Dr. Edmond Locard (1877 – 1966), Locard’s Exchange Principle, which forms the foundation of forensic science, holds that the perpetrator of a crime will bring something to the crime scene and will leave with something from it. At its core, the concept is that every contact leaves a trace and that with contact between two items, there will always be an exchange. To quote Dr. Locard, “It is impossible for a criminal to act, especially considering the intensity of a crime without leaving traces of this presence.”    

Think about it. It makes sense.

Wherever a criminal (or even a totally innocent, law-abiding citizen) goes, he will leave something behind. At the same time, he will also take something back with him. Our criminal (or law-abiding citizen) can leave all sorts of evidence, like fingerprints, footprints, hair, skin, blood, bodily fluids, pieces of clothing—and more. By coming into contact with others, or things, at the location in question, our criminal (or law-abiding citizen) takes part of that location with him when he leaves, whether it’s dirt, hair, or other types of trace evidence.

During an investigation in 1912, a Frenchwoman named Marie Latelle was found dead in her parents’ home. Her boyfriend, Emile Gourbin, claimed he had been playing cards with friends on the night of the murder. Absent evidence to the contrary, police concluded Gourbin was telling the truth.

Not deterred, Dr. Locard looked at Marie’s corpse and questioned Gourbin’s alibi. Locard found clear evidence that Marie had been strangled. He then scraped underneath Gourbin’s fingernails and examined the results under a microscope. Locard noticed a pink dust among the samples, which he calculated must be ladies’ makeup. In that era, makeup was not mass produced. Locard located a chemist who developed a custom powder for Marie. It matched the fingernail scrapings. Ultimately, Gourbin confessed to the murder. He had tricked his friends into believing his alibi by setting the clock in the game room ahead. But he couldn’t trick Locard. When he strangled his girlfriend, he took some of her—skin cells with traces of makeup—with him.

Pretty convincing, right? Most likely, we can all think of “modern day” cases that were solved as a result of Locard’s Exchange Principle. What’s your favorite example?

So, trust me. No matter how hard you try or how well-intentioned you may be, wherever you go you’ll never just leave footprints and you’ll always take away more than just memories. Mystery writers depend on it.